Efficient photocatalytic activity of MnO2-loaded ZrO2/carbon cluster nanocomposite materials under visible light irradiation

Nano-sized ZrO₂/carbon cluster nanocomposite material was successfully prepared by the calcination of Zr(acac)₄/epoxy resin complex in air. The composite material obtained by calcining at 200 °C was treated with hydrogen hexachloroplatinate hexahydrate (H₂PtCl₆) to obtain Pt-loaded materials denoted as Ic₂₀₀Pt'sH's. The Pt-loaded material modified with MnO₂ particles efficiently decompose water into H₂ and O₂ with a [H₂]/[O₂] ratio of 2 under the irradiation of visible light (λ > 460 nm) through the electron transfer process of MnO₂ → carbon clusters → ZrO₂ → Pt.     

Transmission electron microscopy characterization of nanostructured carbon derived from Cr3C2 and Cr(C5H7O2)3

The preparation, characterization and comparison of nanostructured carbons derived by direct chlorination of Cr₃C₂ and Cr(C₅H₇O₂)₃ are reported in this work. Cr₃C₂ precursor was treated at 400 and 900 °C with a reaction time of 1 h. The nanostructure of the products has been characterized in some detail by means of transmission electron microscopy and associated techniques, such as electron energy-loss and X-ray energy dispersive spectroscopies and high-angle annular dark field imaging. Remains of Cr₃C₂ encapsulated in an amorphous carbon shell were observed at 400 °C, whereas carbon with higher ordering degree was produced at 900 °C. In the latter case, the sample can be described as a continuous variation from poorly-stacked graphene-like carbon to graphitic agglomerates. Remains of the reaction by-product, CrCl₃, are detected in the carbon particles, forming monolayers intercalated inside the graphitic agglomerates and amorphous nanoparticles. As a comparison, carbon samples derived from Cr(C₅H₇O₂)₃ were prepared at 300 and 900 °C. They mainly consist of highly disordered carbon, with local graphite-like stacking in the sample prepared at 900 °C.     

Cr2O3 doping in ZnO–0.5 mol% V2O5 varistor ceramics

The effects of the amount of Cr₂O₃ (0.5–4 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO–0.5 mol% V₂O₅ system. The microstructure of the samples consists mainly of ZnO grains with ZnCr₂O₄ and α-Zn₃(VO₄)₂ as the minority secondary phases. The addition of Cr₂O₃ is found to be effective in controlling the abnormal ZnO grain growth often found in V₂O₅-doped ZnO ceramic system, and a more uniform microstructure can be obtained. The varistor performance is also improved as observed from the increase in the non-linear coefficient α of the Cr₂O₃-doped ZnO–V₂O₅ samples. The α value is found to increase with the amount of Cr₂O₃ for up to 3 mol% Cr₂O₃ content. Further increase in Cr₂O₃ is found to cause a decrease in the α value. The highest α value of 28.9 is obtained for the ZnO–0.5 mol% V₂O₅–3 mol% Cr₂O₃ sample.     

Investigation on electrochemical reduction process of Nb2O5 powder in molten CaCl2 with metallic cavity electrode

The direct electrochemical reduction process of Nb₂O₅ powder was investigated by cyclic voltammetry and constant potential electrolysis with a novel metallic cavity electrode in molten calcium chloride at 850 °C. The products of both constant potential and constant voltage electrolysis were characterized by XRD, SEM and EDX. CaNb₂O₆ was formed upon addition of solid Nb₂O₅ into molten CaCl₂ when CaO was present. During the electrolysis solid Nb₂O₅ was reduced to various niobium oxides of lower oxidation states, including some composite oxides, and then was converted completely to metallic niobium near −0.35 V (vs. Ag/AgCl), which was more positive than the reduction potential of Ca²⁺. Constant potential electrolysis was applied at the potentials near the reduction current peaks derived from the cyclic voltammetry curves, and cell voltages were monitored. The voltage was near 2.4 V when the oxide was metallized at −0.35 V (vs. Ag/AgCl). Nb₂O₅ pellet could be used to prepared metallic niobium at cell voltage 2.4 V in a larger electrolysis bath filled with calcium chloride at 850 °C. The experiment results further demonstrated the direct electrochemical reduction mechanism of Nb₂O₅ powder in a molten system.     

Microstructural, electrical and magnetic properties of multiferroic CoFe2O4/0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 nanocomposite thin films

Bilayered CoFe₂O₄/0.68Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ nanocomposite films are successfully prepared on Pt/Ti/SiO₂/Si substrate via simple sol-gel process. X-ray diffraction result reveals that there exists no chemical reaction or phase diffusion between the CoFe₂O₄ and 0.68Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ phases. The microstructure is characterized by scanning/transmission electron microscopy (STEM). The composite thin films exhibit both strong ferroelectric and ferromagnetic responses at room temperature. The maximal magnetoelectric coupling coefficient of the nanocomposite films reaches up to 25 mV/cm Oe, occurs at a lower bias magnetic field (H_dc) of 550 Oe.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Fixed- and fluidised-bed synthesis of coiled carbon fibres on an in situ generated H2S-modified Ni/Al2O3 catalyst from a NiSO4/Al2O3 precursor

NiSO₄/Al₂O₃ and NiO/Al₂O₃ catalyst precursors were formed by calcination of NiSO₄·6H₂O/Al₂O₃ at 500 and 800 °C, respectively. The catalyst precursor was reduced under H₂ and N₂ and then reacted under C₂H₂, H₂ and N₂ at 650 °C. Coiled carbon fibres were formed in fixed- and fluidised-bed reactors using the NiSO₄/Al₂O₃ catalyst precursor. Thermodynamic modelling using an infinite equilibrium stage construction predicted complete reduction of NiSO₄ to Ni and simultaneous H₂S formation occurs in both fixed- and fluidised-bed systems. XRD measurements confirmed that Ni was the only catalytically active crystalline species present at concentrations >0.5 wt.% (XRD detection limit) post-reduction, however XRF and XPS measurements additionally detected the presence of small quantities (<0.9 wt.% S) of S species. S is adsorbed onto the Ni surfaces during reduction when H₂S is released and dissociates on the Ni surface. Non-coiled carbon fibres produced on the Ni/Al₂O₃ catalyst formed from the NiO/Al₂O₃ precursor demonstrated that modification of Ni/Al₂O₃ with S is required for coiled carbon fibre synthesis.     

Synthesis of Zn3Nb2O8 ceramics using a simple and effective process

Synthesis of Zn₃Nb₂O₈ ceramics using a simple and effective reaction-sintering process was investigated. The mixture of ZnO and Nb₂O₅ was pressed and sintered directly without any prior calcination. Single-phase Zn₃Nb₂O₈ ceramics could be obtained. Density of these ceramics increased with soaking time and sintering temperature. A maximum density 5.72 g/cm³ (99.7% of the theoretical density) was found for pellets sintered at 1170 °C for 2 h. Pores were not found and grain sizes >20 μm were observed in pellets sintered at 1170 °C. Abnormal grain growth occurred and grains >50 μm could be seen in Zn₃Nb₂O₈ ceramics sintered at 1200 °C for 2 h and 1200 °C for 4 h. Reaction-sintering process is then a simple and effective method to produce Zn₃Nb₂O₈ ceramics for applications in microwave dielectric resonators.     

Amorphous Ru1−yCryO2 loaded on TiO2 nanotubes for electrochemical capacitors

Amorphous Ru_1−yCr_yO₂/TiO₂ nanotube composites were synthesized by loading different amount of Ru_1−yCr_yO₂ on TiO₂ nanotubes via a reduction reaction of K₂Cr₂O₇ with RuCl₃·nH₂O at pH 8, followed by drying in air at 150 °C. Cyclic voltammetry and galvanostatic charge/discharge tests were applied to investigate the performance of the Ru_1−yCr_yO₂/TiO₂ nanotube composite electrodes. For comparison, the performance of amorphous Ru_1−yCr_yO₂ was also studied. The results demonstrated that the three dimensional nanotube network of TiO₂ offered a solid support structure for active materials Ru_1−yCr_yO₂, allowed the active material to be readily available for electrochemical reactions, and increased the utilization of active materials. A maximum specific capacitance 1272.5 F/g was obtained with the proper amount of Ru_1−yCr_yO₂ loaded on the TiO₂ nanotubes.     

Preparation and characterization of Cr2O3-TiO2-Al2O3-V2O5 green pigment

Green pigments with high near infrared reflectance based on a Cr₂O₃-TiO₂-Al₂O₃-V₂O₅ composition have been synthesized. Cr₂O₃ was used as the host component and mixtures of TiO₂, Al₂O₃ and V₂O₅ were used as the guest components. TiO₂, Al₂O₃, and V₂O₅ were mixed into 39 different compositions. The spectral reflectance and the distribution of pigment powder were determined using a spectrophotometer and a scanning electron microscope, respectively. It was found that a pigment powder sample S9 with a Cr₂O₃-TiO₂-Al₂O₃-V₂O₅ composition of 80, 4, 14 and 2 wt%, respectively, gives a maximum near infrared solar reflectance of 82.8% compared with 49.0% for pure Cr₂O₃. The dispersion of pigment powders in a ceramic glaze was also studied. The results show that the pigment powder sample S9 is suitable for use as a coating material for ceramic-based roofs.     

Phase stability and electric conductivity of Er2O3-Nb2O5 co-doped Bi2O3 electrolyte

Two oxides, Er₂O₃ and Nb₂O₃, are used to stabilize delta-phase Bi₂O₃ used as electrolyte of solid oxide fuel cell. Optimization of dopant ratio and total doping concentration (TDC) is determined by X-ray diffraction, and successfully reduce the TDC (Er + Nb) to 10-15 mol.%. Conductivities of different compositions are measured by two-probe method. The results show that highest conductivity appears at the minimum doping concentrations. Phase stability of ENSB samples with Er/Nb ratio of 2/1 and TDC of 10-20 mol.% at 650 °C up to 300 h is analyzed showing two newly formed (alpha- and gamma-) phases in the samples. Degradation of conductivity at 650 °C is studied in detail by DTA and TEM. The abnormity of lattice contraction of delta-phase is discussed.     

Solvent-assisted molten salt process: A new route to synthesise α-Fe2O3/C nanocomposite and its electrochemical performance in lithium-ion batteries

Nanostructured α-Fe₂O₃ was synthesised by a simple molten salt process using FeCl₂·4H₂O as a starting material and LiNO₃-LiOH·H₂O-H₂O₂ as a eutectic mixture at 300 °C. To synthesise α-Fe₂O₃/C composite, both α-Fe₂O₃ and malic acid were dispersed together in toluene, where malic acid was used as a carbon source. The morphology and microstructure of both compounds were confirmed by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. Electrochemical testing, including constant current charge-discharge and cyclic voltammetry (CV), was carried out. The α-Fe₂O₃/C composite anode exhibited much better electrochemical performance than the bare α-Fe₂O₃. The discharge capacities of the composite were measured to be 2112 mAh g⁻¹ at C/2 after 100 cycles and 584 mAh g⁻¹ at 20 C after 10 cycles. The superior electrochemical performance of α-Fe₂O₃/C composite can be mainly attributed to the combined effects of the nanostructure, the carbon layering on the α-Fe₂O₃ nanoparticles, and the porous ultra-fine carbon matrix, where the three factors would contribute to provide high electronic conductivity, reduce the traverse time of electrons and lithium-ions, and could also prevent high volume expansion of the anode film during cycling. Our results indicate that the prepared α-Fe₂O₃/C nanocomposite is a very promising anode material for Li-ion power batteries.     

Catalytic dehydration of methanol to dimethyl ether over modified γ-Al2O3 catalyst

A γ-Al₂O₃ catalyst was modified with metal oxide (Nb₂O₅) in order to improve its activity and stability for dimethyl ether (DME) synthesis from methanol. A series of modified γ-Al₂O₃ catalysts were prepared with Nb₂O₅ and characterized by X-ray diffraction and temperature programmed desorption of ammonia. Results showed that the γ-Al₂O₃ catalyst containing 10 wt.% of Nb₂O₅ exhibited the highest surface area among the modified ones. The number of acid sites of the modified catalysts was increased by the Nb₂O₅ modification. In the chemical reaction of DME synthesis, it was found that the Nb₂O₅ modified γ-Al₂O₃ catalyst exhibited a higher activity in the low temperature region (240 °C-260 °C) and a higher activity than did the untreated γ-Al₂O₃ catalyst.     

Microwave dielectric properties of Ba3Ti4−x(Zn1/3Nb2/3)xNb4O21 for low temperature co-fired ceramics

The effects of substitution of (Zn_1/3Nb_2/3) for Ti on the sintering behavior and microwave dielectric properties of Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (0 ≤ x ≤ 4) ceramics have been investigated. The dielectric constant (?_r) and the temperature coefficient of the resonant frequency (τ_f) of Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ ceramics decreased with increasing x. However, the Q × f values enhanced with the substitution of (Zn_1/3Nb_2/3) for Ti. It was found that a small amount of MnCO₃-CuO (MC) and ZnO-B₂O₃-SiO₂ (ZBS) glass additives to Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (x = 2) ceramics lowered the sintering temperature from 1250 to 900 °C. And Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (x = 2) ceramics with 1 wt% MC and 1 wt% ZBS sintered at 900 °C for 2 h showed excellent dielectric properties: ?_r = 53, Q × f = 14,600 GHz, τ_f = 6 ppm/°C. Moreover, it has a chemical compatibility with silver, which made it as a promising material for low temperature co-fired ceramics technology application.     

Luminescent properties of Sr2B2O5: Tm,Na blue phosphor

A novel blue phosphor, Sr₂B₂O₅: Tm³⁺, Na⁺ for white light-emitting diodes (W-LEDs) was prepared by solid-state synthesis and its structure and luminescence properties were investigated. This phosphor can be effectively excited within the broad near ultraviolet (NUV) wavelength region, from 340 nm to 370 nm, and exhibits a satisfactory blue performance. The emission peaks are observed at 457 nm (blue) and 475 nm (blue), due to the respective transitions of ¹D₂→³F₄ and ¹G₄→H₆. Seven mole percent of doping concentration of Tm³⁺ was shown to be optimal. Concentration quenching occurs when Tm³⁺ concentration is beyond 7 mol%, its mechanism being explained by dipole–dipole interaction of Tm³⁺ and being confirmed by decay property measurements. We have made a deep analysis on the effect of charge compensation reagent on luminescence intensity. Good blue emissions with the CIE chromaticity coordinates (0.173, 0.165) could be achieved. Our results suggest that the Sr₂B₂O₅: Tm³⁺, Na⁺ phosphor is a potential blue-emitting material.     

Low-temperature synthesis of ZnNb2O6 powders via hydrothermal method

ZnNb₂O₆ powder was successfully synthesized via hydrothermal method with Nb₂O₅ and Zn(NO₃)₂·6H₂O as raw materials and cyclohexane as solvent. Phase composition, morphology, and chemical composition were determined via a combination of XRD, SEM, TEM and EDS techniques. The effects of synthesis temperature and reaction time on phase composition and particle morphology were investigated in this paper. The results showed that fine ZnNb₂O₆ powders could be obtained at a hydrothermal temperature of 190 °C or above under different reaction time.     

A facile molten salt route to K2Nb8O21 nanoribbons

Single-crystalline K₂Nb₈O₂₁ nanoribbons with width of 100–500 nm and thickness of ca. 30 nm, and length up to tens of micron, have been successfully synthesized by simply calcining Nb₂O₅ powders in molten KCl, and characterized with XRD, SEM, SEM, HRTEM and selected-area electron diffraction technique. The growth direction of the obtained K₂Nb₈O₂₁ nanoribbons was determined to be the 〈1 0 0〉 crystallography direction.     

Fabrication and characterisation of Cr and Co doped Bi1.5Zn0.92Nb1.5O6.92 pyrochlores

Cr and Co doped Bi_1.5Zn_0.92Nb_1.5O_6.92 pyrochlore ceramics were produced by solid state mixing of oxides. Cr and Co were doped into the Nb and Nb-Zn sites considering the compositions of Bi_1.5Zn_0.92Nb_1.5−xCr_xO_6.92−x, (Bi_1.5Zn_0.46)(Zn_0.46−3x/6Nb_1.5−3x/5Cr_x)O_6.92−x/2 for Cr doping and Bi_1.5Zn_0.92Nb_1.5−3x/5Co_xO_6.92, (Bi_1.5Zn_0.46)(Zn_0.46−3x/6Nb_1.5−3x/5Co_x)O_6.92−x/2 for Co doping. The solubility limit of Cr in BZN was higher than that of Co and the solubility limit increased when doping was made both into Nb and Zn sites. The second phases appeared when x > 0.2 for Cr and x > 0.15 for Co doping into the Nb-Zn sites. Simultaneous Cr doping into the Nb- and Zn-sites of BZN pyrochlore gave higher dielectric constant than doping into the Nb-site of pyrochlore. However, Co doping into the Nb- and Zn-sites and only into the Nb-site of BZN gave identical dielectric results in the range of 202-218. The temperature coefficient of dielectric constant decreased with Cr doping and increased with Co doping.     

Electrocatalyst materials for fuel cells based on the polyoxometalates—K7 or H7[(P2W17O61)Fe(H2O)] and Na12 or H12[(P2W15O56)2Fe4(H2O)2]

Free acids of the iron substituted heteropoly acids (HPA), H₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (HFe1) and H₁₈[(P₂W₁₅O₅₆)₂Fe^III₂(H₂O)₂] (HFe2) were prepared from the salts K₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (KFe1) and Na₁₂[(P₂W₁₅O₅₆)₂Fe^III₄(H₂O)₂] (NaFe4), respectively. The iron-substituted HPA were adsorbed on to XC-72 carbon based GDLs to form HPA doped GDEs after water washing with HPA loadings of ca. 1 μmol. The HPA was detected throughout the GDL by EDX. Solution electrochemistry of the free acids are reported for the first time in sulfate buffer, pH 1-3. The hydrogen oxidation reaction was catalyzed by KFe1 at 0.33 V, with an exchange current density of 38 mA/cm². Moderate activity for the oxygen reduction reaction was observed for the iron substituted HPA, which was dramatically improved by selectively removing oxygen atoms from the HPA by cycling the fuel cell cathode under N₂ followed by reoxidation to give a restructured oxide catalyst. The nanostructured oxide achieved an OCV of 0.7 V with a Tafel slope of 115 mV/decade. Cycling the same catalysts in oxygen resulted in an improved catalyst/ionomer/carbon configuration with a slightly higher Tafel slope, 128 mV/decade but a respectable current density of 100 mA/cm² at 0.2 V.     

Direct synthesis of H2O2 acid solutions on carbon cathode prepared from activated carbon and vapor-growing-carbon-fiber by a H2/O2 fuel cell

Direct synthesis of H₂O₂ acid solutions was studied using a gas-diffusion cathode prepared from activated carbon (AC), vapor-growing-carbon-fiber (VGCF) and poly-tetra-fluoro-ethylene (PTFE) powders, with a new H₂/O₂ fuel cell reactor. O₂ reduction to H₂O₂ was remarkably enhanced at the three-phase boundary (O₂(g)-electrode(s)-acid(l)) at the [AC + VGCF] cathode. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk acid solutions were essential for H₂O₂ accumulation. Synergy of AC and VGCF was observed for the H₂O₂ formation. RRDE and cyclic voltammetry studies indicated that the surface of AC functioned as the active phase for O₂ reduction to HO₂, and VGCF functioned as an electron conductor and a promoter to convert HO₂ to H₂O₂. A maximum H₂O₂ concentration of 353 mM (1.2 wt%) was accomplished under short-circuit conditions (current density 12.7 mA cm⁻², current efficiency 40.1%, geometric area of cathode 1.3 cm², reaction time 6 h).