Xudong Zhao

Controlled growth of mesoporous ZnCo2O4 nanosheet arrays on Ni foam as high-rate electrodes for supercapacitors

Mesoporous ZnCo2O4 nanosheet arrays are uniformly grown on Ni foam via a facile two-step method and directly use it as an integrated electrode for supercapacitors. The ZnCo2O4 nanosheet arrays/Ni foam electrode exhibits ultrahigh capacitance even with high current densities and excellent cycling stability after long term tests because of its unique nanostructure.

A facile template-free approach for the solid-phase synthesis of CoS2 nanocrystals and their enhanced storage energy in supercapacitors

Sphere-like CoS2 nanocrystals (NCs) with porous surfaces were prepared by a modified molten-salt synthesis (MSS) method under mild reaction conditions. Thiourea was used as a reactant, flux and structure-directing agent in the synthesis of CoS2 NCs, which led to mild reaction conditions, pure product and a porous spherical surface. The increase of reaction temperature and molar ratio of cobalt nitrate to thiourea results in larger CoS2 NCs and the change of their morphologies from spheroidal to angular. The synthesis mechanism of CoS2 nanocrystals includes three steps which are the formation of sphere-like CoS2 NCs at the early reaction stage, the formation of a porous structure on the CoS2 NC surface and the aggregation of NCs. The specific surface area of CoS2 NC electrodes reached 29.30 m2 g−1 which results in a specific capacitance as high as 654 F g−1. These results demonstrate that CoS2 NCs can be produced on a large scale through a simple solid-phase synthesis pathway and that they will be a promising electrode material for supercapacitors.

Hydrothermal approach and luminescent properties for the synthesis of orthoniobates GdNbO4:Ln3+ (Ln = Dy, Eu) single crystals under high-temperature high-pressure conditions

Single crystal GdNbO4:Ln3+ (Ln = Dy, Eu) phosphors were prepared via a high-temperature high-pressure hydrothermal procedure at 650 °C under autogenous pressure. X-ray diffraction, field emission scanning electron microscopy, photoluminescence, Raman and XPS were utilized to characterize the as-synthesized phosphors. XRD reveals that the samples begin to crystallize at 550 °C and a pure GdNbO4 phase can be obtained at 650 °C. FE-SEM images indicate that GdNbO4:Ln3+ (Ln = Dy, Eu) samples consist of fine sheets with a size of 50–100 μm. Under UV excitation, the GdNbO4:Eu3+ and GdNbO4:Dy3+ phosphors showed the characteristic emissions of Eu3+ are 5D0 → 7FJ (J = 0, 1, 2, 3, 4), and Dy3+ (4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions), respectively. The preparation method presented here dramatically lowered the traditional temperature of 2300 °C or above in the Czochralski method. The GdNbO4:Dy3+ single crystals showed a bright white emission under different excitation wavelengths with a relatively high quantum yield of 21.7%. The GdNbO4:0.05Eu3+ exhibited excellent bright red luminescence at 612 nm under near-UV excitation, narrowed emission spectra, room temperature luminescence lifetimes of milliseconds and maximum quantum efficiencies of 43.2%.

Rapid microwave synthesis of δ-MnO2 microspheres and their electrochemical property

The δ-MnO2 microspheres as well as the δ-MnO2 microspheres/α-MnO2 nanorods mixture were synthesized by reduction of KMnO4 under microwave irradiation. The morphology and structure of the as-prepared products were characterized by powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer–Emmett–Teller (BET) measurements and Raman spectra. Electrochemical performances of the samples were examined using cyclic voltammetry (CV) and galvanostatical discharge–charge test in Na2SO4 and Li2SO4 aqueous electrolyte, respectively. It was found that the nearly ideal rectangular shapes of the CV curves show that these samples exhibited good pseudo-capacitor properties in the voltage range from 0.0 to 1.0xa0V. The test of long-term cycling stability indicated that two samples all have a reasonably good stability. However, the δ-MnO2 microspheres possessed a lower specific surface area while a higher supercapacitance than those of the δ-MnO2 micropheres/α-MnO2 nanorods mixture did. The presence of a few fraction α-MnO2 impurities reduces the specific capacitance of δ-MnO2 electrode. Because of the crystal structure and morphologies are two such important parameters that have to be taken into account in this work.

High-temperature, high-pressure hydrothermal synthesis, crystal structure and photoluminescent properties, of K3[Gd1−xTbxGe3O8(OH)2] (x = 0, 0.3, 0.1, 1)

A family of 2D-layered lanthanide germanates K3[Gd1−xTbxGe3O8(OH)2] (x = 0, 0.3, 0.1, and 1), have been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, photoluminescence, IR spectra, and Energy-Dispersive Spectroscopy (EDS). The X-ray powder diffraction patterns of these compounds reveal that they are isostructural. The single-crystal X-ray diffraction analysis of K3[GdGe3O8(OH)2] reveals that it is a 2D-layered [LnGe3O8(OH)2]n3n− anionic framework which is built up from GeO4H/GeO4 tetrahedra and GdO6 octahedra by sharing vertex O atoms. K+ ions locate in the free void space to achieve the charge balance of the framework. A sample containing only Tb3+ emits mainly from one transition, 5D4 → 7F5 (552 nm). Mixed lanthanide samples, K3[Gd1−xTbxGe3O8(OH)2] (x = 0.3, and 0.1), have also been prepared and efficient Gd → Tb energy transfer has been observed.

Synthesis and characterization of multipod frameworks of Cu2O microcrystals and Cu7S4 hollow microcages

A variety of multipod frameworks of Cu2O microcrystals have been prepared through careful control of the ratios of n-butyl alcohol to water, and novel 14-pod Cu2O frameworks are first reported. Moreover, template-assisted synthesis of multipod frameworks of Cu7S4 microcages using the obtained Cu2O microcrystals as sacrificial templates is reported.

Microwave-assisted synthesis of Cu2O microcrystals with systematic shape evolution from octahedral to cubic and their comparative photocatalytic activities

Cuprous oxide (Cu2O) microcrystals with systematic shape evolution were successfully synthesized via a facile microwave-assisted heating technique. The monodispersed Cu2O was synthesized using copper acetate as a starting material, ethylene diamine tetraacetic acid (EDTA) as a reducing agent and surface-regulating agent and a mixture of water and n-butyl alcohol as reaction solvent. Various morphologies of Cu2O microcrystals, including octahedral, truncated octahedral, cuboctahedral, truncated cubic and cubic microcrystals, were obtained by altering the volume ratio of n-butyl alcohol to water. The morphologies and optical properties of the synthesized Cu2O microcrystals were characterized by XRD, SEM, TEM, HRTEM, SADE and UV-Vis/DRS. The growth mechanism of these crystals was thereby proposed. The volume ratio of n-butyl alcohol to water in the reaction medium was a critical factor in precisely controlling the morphologies of the microcrystals. Furthermore, their comparative photocatalytic activities for the degradation of methyl orange were tested.

Selective synthesis of cubic and hexagonal phase of CuInS2 nanocrystals by microwave irradiation

Metastable cubic zincblende and hexagonal wurtzite CuInS2 nanocrystals were successfully synthesized by a facile microwave radiation method. The morphology, structure and phase composition of the as-prepared products were examined, and the results demonstrated that high-purity and uniform CuInS2 nanocrystals were obtained. Further investigation revealed a structural evolution process from cubic zincblende to hexagonal wurtzite with increasing volume ratio of ethylenediamine and ethanol from 1:30 to 1:1 at 160 °C. In addition, the optical absorption properties of samples were also studied. It was found that the as-synthesized cubic and hexagonal CuInS2 nanocrystals had band gaps of 1.503 and 1.470 eV, respectively. Thanks to the superior optical absorption properties for visible light, CuInS2 nanocrystals may have potential applications in various nanostructured optoelectronic devices. The possible formation mechanism of the product, namely “phase transformation”, was systematically studied.

The synthesis of hierarchical ZnCo2O4@MnO2 core–shell nanosheet arrays on Ni foam for high-performance all-solid-state asymmetric supercapacitors

Hierarchical ZnCo2O4@MnO2 core–shell nanosheet arrays were successfully synthesized on Ni foam via a facile hydrothermal method followed by a calcination process. The fabricated ZnCo2O4@MnO2 electrode exhibited a specific capacitance as high as 2170.00 F g−1 (2.60 F cm−2) at a current density of 3 mA cm−2 in a 1 M KOH solution. Furthermore, an all-solid-state asymmetric supercapacitor (ASC) device fabricated with the as-prepared ZnCo2O4@MnO2 as the positive electrode and activated carbon (AC) as the negative electrode in the PVA/KOH gel electrolyte achieved a high energy density of 29.41 W h kg−1 at a power density of 628.42 W kg−1 and retained 95.3% of its initial capacitance after 3000 cycles at a high current density of 10 mA cm−2. With the smart design and remarkable electrochemical properties, the hierarchical ZnCo2O4@MnO2 core–shell nanosheet arrays on Ni foam demonstrated great potential for further applications in the energy storage field.

Microwave synthesis and photocatalytic activity of Tb3+ doped BiVO4 microcrystals

Tb(3+) doped BiVO4 has been successfully synthesized by a simple microwave-assisted hydrothermal method at 140°C for 30min. The structure, morphology and optical property of the Tb(3+) doped BiVO4 products have been systematically investigated. This study indicates that the incorporation of Tb(3+) could induce the conversion of structure from monoclinic to tetragonal for BiVO4. Furthermore, the as-obtained Tb(3+) doped BiVO4 samples showed an obvious morphological change: the hollow square rod-like BiVO4 crystal gradually changed to spindle-like crystal. The Tb(3+) doped BiVO4 products exhibited extraordinary photocatalytic activity for Methylene Blue (MB) degradation under visible light irradiation. The doped BiVO4 at a molar ratio of 2at% (Tb and Bi) with a mixture of monoclinic and tetragonal phases showed and prominent photocatalytic degradation rate, which reached 99.9% in 120min. The results suggest that the differences in the photocatalytic activity of these BiVO4 crystals with different Tb(3+) doping concentrations can be attributed to the change of crystalline phases, and the coexistence of the monoclinic/tetragonal phases in BiVO4 products, which improve the efficient charge separation and transportation.