Dickon H. L. Ng

High Efficiency Photocatalysis for Pollutant Degradation with MoS2/C3N4 Heterostructures

Porous graphitic carbon nitride was synthesized by controllable thermal polymerization of urea in air. Their textural, electrical, and optical properties were tuned by varying the heating rate. The presence of proper residual oxygen in carbon nitride matrix had enhanced light absorption and inhibited the recombination of charge carriers. Furthermore, the MoS2 nanosheets were coupled into the carbon nitride to form MoS2/C3N4 heterostructures via a facile ultrasonic chemical method. The optimized MoS2/C3N4 heterostructure with 0.05 wt % MoS2 showed a reaction rate constant as high as 0.301 min(-1), which was 3.6 times that of bare carbon nitride. As analyzed by SEM, TEM, UV-vis absorption, PL and photoelectrochemical measurements, intimate contact interface, extended light response range, enhanced separation speed of charge carriers, and high photocurrent density upon MoS2 coupling led to the photocatalytic promotion of the MoS2/C3N4 heterostructures. In this architecture, MoS2 served as electron trapper to extend the lifetime of separated electron-hole pairs. Meanwhile, the accumulated holes on the surface of carbon nitride oxidized the organic dye directly, which was a predominant process in the photodegradation of organic pollutants in water treatment. The promotional mechanisms and principles reported here would have great significance in heterogeneous photocatalysis.

Ag nanoparticle decorated nanoporous ZnO microrods and their enhanced photocatalytic activities.

Nanostructured Ag nanoparticles (Ag-NPs)/nanoporous ZnO micrometer-rods (n-ZnO MRs) have been synthesized by a two-step method. The n-ZnO MRs was initially prepared by solvothermal-assisted heat treatment. The rods had the diameter ranged from 90 to 150 nm and length between 0.5 and 3 μm. They were found to be porous and were composited of ZnO nanopartiles with size of about 20 nm. In the second stage, Ag-NPs with a diameter of 20-50 nm were anchored onto the surface of the as-prepared n-ZnO MRs by a photoreduction method. The Ag-NPs/n-ZnO MRs were evaluated for their ability to degrade methylene blue (MB) solution under visible to ultraviolet (UV) light irradiation. The rate of degradation of the as-prepared Ag-NPs/n-ZnO MRs was more than twice and nearly 5.6 times faster than that of using bare n-ZnO MRs under the UV and solar light irradiation, respectively. The formation of Schottky barriers in the regions between the Ag-NPs and n-ZnO MRs had improved the charge separation and consequently enhanced the efficiency of the degradation process. Moreover, the as-prepared hybrid structure exhibited high photostability, and 98% of degradation efficiency could be maintained even after being used five times. This endurance was attributed to the retardation of photocorrosion of ZnO as a result of the low concentration of surface defects in the as-prepared n-ZnO MRs. It also minimized the surface defects of the as-prepared n-ZnO MRs and consequently further inhibited the photocorrosion of ZnO when the deposited Ag-NPs were much more inclined to combine with the chemisorbed oxygen.

Microwave-Assisted Fabrication of Nanoparticulate TiO2 Microspheres for Synergistic Photocatalytic Removal of Cr(VI) and Methyl Orange

High yield production of micro/nanostructured nanoparticulate TiO2 microspheres (NTMs) via a facile microwave-assisted hydrothermal approach was investigated. The rapid and uniform microwave heating could reduce the reaction time to 30 min, an order of magnitude shorter than that of conventional hydrothermal methods. The characterization data confirmed that the resultant NTMs were highly uniform in size, having an average diameter of ∼0.5 μm. The obtained NTMs were found to be constructed by well-crystallized anatase phase nanoparticles ranging from 5 to 10 nm that can be readily controlled by the microwave radiation temperature. Nitrogen sorption isotherm analysis revealed that the obtained NTMs possessed abundant mesoporous structures with a high specific surface area of 124 m(2) g(-1). An in situ self-aggregation formation process under controllable pH in presence of urea was proposed. The results obtained from the application of NTMs for simultaneous photocatalytic decontamination of Cr(VI) and methyl orange (MO) demonstrated a strong synergistic effect that dramatically enhanced both Cr(VI) reduction and MO oxidation removal efficiencies. This work not only enriched the synthesis methods of the micro/nanostructured TiO2, but also provided a new means to improve the photocatalytic efficiency via structural-induced synergistic effect, applicable to the other catalysis systems.

A Versatile Method for Controlled Synthesis of Porous Hollow Spheres

A versatile method was developed to synthesize nickel silicate, silica, and silica-nickel composite porous hollow spheres by using silica spheres as templates. In the preparation, silica spheres were treated with a mixture of NiSO(4)·6H(2)O and NH(3)·H(2)O. The nickel-based ingredient reacted with the silica to form a shell while the alkaline solution could remove the silica core, thus forming the nickel silicate hollow spheres. After these spheres were further treated with hydrogen in reduction or with HCl in etching, they became silica-nickel hollow spheres or silica hollow spheres, respectively. The sizes of these hollow spheres depended on the concentration of the precursor. Our investigation also found that their surface properties or magnetic properties could be tailored by adjusting the fabrication parameters.

Formation of nanostructured eutectic network in α-Al2O3 reinforced Al–Cu alloy matrix composite

We report the fabrication and characterization of an Al-Cu alloy matrix composite, which is reinforced by alpha-Al-2-O-3 particles and nanometer-sized (nm-sized) lamellar eutectic. The composite was fabricated by sintering and rapid quenching of an Al-20wt.%CuO sample. We had performed differential thermal analysis on the sample, and found that the reaction between Al and CuO took place between 580 and 700 degreesC. Results from scanning and transmission electron microscopies indicated that amorphous Al2O3 was initially formed in the reaction. It was then crystallized and transformed to the more stable alpha-Al2O3 particles at higher temperature. When the sintered sample was cooled down from 1000 degreesC, the two-phase Al(Cu)-Al2Cu eutectic was formed. The size and the distribution of the eutectic network strongly depended on the rate of cooling. The final sintered product contains alpha-Al2O3 particles and Al(Cu)-AlCu eutectic, which are embedded in the AI(Cu) matrix. In comparison, the eutectic network in the oil-quenched sample is distributed more evenly and is finer in size, with lamellar spacing as small as 200-300 nm, than that of the furnace-cooled sample. The Vickers hardness value of the oil-quenched sample has an average of 123, which is 50% higher than that of the furnace-cooled sample

Synthesis and characterization of nanostructured Fe3O4 micron-spheres and their application in removing toxic Cr ions from polluted water

We present a simple and effective method for the synthesis of nanostructured Fe(3)O(4) micron-spheres (NFMSs) by annealing hydrothermally formed FeCO(3) spheres in argon. The phase structure, particle size, and magnetic properties of the product have been characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and by means of a superconducting quantum interference device (SQUID). The results have shown that the as-obtained NFMSs have a diameter of about 5 μm and are composed of nanometer-sized porous lamellae. The NFMSs have a large specific surface area (135.9 m(2) g(-1)), reductive Fe(2+) incorporated into their structure, and intense magnetic properties. These properties suggest that NFMSs have potential application in removing toxic Cr(6+) ions from polluted water. At 25°C, each gram of NFMSs product can remove 43.48 mg of Cr(6+) ions, as compared to just 10.2 mg for nanometer-sized Fe(3)O(4) and 1.89 mg for micron-sized Fe(3)O(4). The enhanced removal performance can be ascribed to the structural features. Moreover, the Cr(6+) ion removal capacity of the NFMSs can reach up to 71.2 mg g(-1) at 50°C. The influencing parameters in the removal of Cr(6+) ions, such as contact time, pH, and temperature, have been evaluated. The Cr(6+)-removal mechanism has been investigated. We have found that the NFMSs product not only serves as an effective adsorbent to remove toxic Cr(6+) ions from polluted water, but also as an effective reductant in reducing the adsorbed toxic Cr(6+) ions to much less toxic Cr(3+) through the Fe(2+) incorporated into its structure.

Template-free hydrothermal synthesis of hexagonal ZnO micro-cups and micro-rings assembled by nanoparticles

The hexagonal ZnO micro-cups and micro-rings assembled by nanoparticles have been successfully synthesized via a template-free hydrothermal synthetic method, using only one reactant Zn(CH3COO)2·2H2O, which not only serves as the zinc source, but also provides an effective etchant CH3COOH that plays a strategic role in the formation of these ZnO hollow structures.

The protective coatings of NdFeB magnets by Al and Al(Fe)

NdFeB permanent magnets are coated by two different methods, dip coating and evaporation coating, with Al. It is found that an intermetallic reaction has occurred and formed a dominant phase of FeAl2.5Bx at the interface between the Al coating and the magnet material. Such reaction enhances strong bonding between the coating and the magnet. When Al(Fe) is used for the coating, no detectable intermetallic phase is observed, and therefore the amount of magnet material is conserved. We have studied the effect of the intermetallic phase on corrosion protection of the magnet. It is found that this phase is less resistive towards HNO3 solution than pure Al; but it is more resistive towards NaOH and NaCl. The investigation of the changes of microstructures of the coatings under different tempering conditions have also been carried out. The results show that the heat treatment at 500 °C for 10 min for a ∼ 2 μm thick Al coating by evaporation can produce an optimal layer of intermetallics. If the temperature is to...

Colossal magnetoresistance in cluster glass-like insulator La0.67Sr0.33(Mn0.8Ni0.2)O3

Substitution of Ni for Mn in La0.67Sr0.33MnO3 (LSMO) lowers the Curie temperature TC from 365 K for LSMO to 194 K for La0.67Sr0.33(Mn0.8Ni0.2)O3, which exhibits a cluster glass-like state. The oxide is insulating under both zero field and 60 kOe, but application of the magnetic field induces colossal magnetoresistance (CMR) especially at low temperature. Far below TC, the compound’s field dependence of resistivity has a very similar shape with that of metallic perovskite manganite at relatively high temperature when the spin fluctuation grows stronger. The results indicate that the ferromagnetic superexchange between Ni and Mn, is helpful for the overall ferromagnetic exchange components just overcoming the generic antiferromagnetic exchange components, which leads to the presence of the cluster glass-like state. The reduction of the magnetic disorder and the suppression of the spin fluctuation in Mn–O layer by the external magnetic field are suggested to explain the CMR effect in this insulating compoundSubstitution of Ni for Mn in La0.67Sr0.33MnO3 (LSMO) lowers the Curie temperature TC from 365 K for LSMO to 194 K for La0.67Sr0.33(Mn0.8Ni0.2)O3, which exhibits a cluster glass-like state. The oxide is insulating under both zero field and 60 kOe, but application of the magnetic field induces colossal magnetoresistance (CMR) especially at low temperature. Far below TC, the compound’s field dependence of resistivity has a very similar shape with that of metallic perovskite manganite at relatively high temperature when the spin fluctuation grows stronger. The results indicate that the ferromagnetic superexchange between Ni and Mn, is helpful for the overall ferromagnetic exchange components just overcoming the generic antiferromagnetic exchange components, which leads to the presence of the cluster glass-like state. The reduction of the magnetic disorder and the suppression of the spin fluctuation in Mn–O layer by the external magnetic field are suggested to explain the CMR effect in this insulating compound

Nitrogen-doped carbon nanofibers with effectively encapsulated GeO2 nanocrystals for highly reversible lithium storage

Germanium dioxide is a promising high-capacity anode material for lithium-ion batteries, but it usually exhibits poor cycling stability due to its large volume change during the lithiation/delithiation process. In this paper, homogeneous dispersive GeO2 nanocrystals encapsulated by nitrogen-doped carbon nanofibers (GeO2-CNFs) were prepared by a facile electrospinning technology. The GeO2-CNF anode material exhibited remarkable capability at a high rate and excellent cycling stability (1031 mA h g−1 at 100 mA g−1 after 200 cycles). The good electrochemical performance could be attributed to the fact that ultra-uniform CNFs act as a blocking layer and a buffer layer to effectively hold GeO2 nanocrystals and facilitate the formation of a stable solid electrolyte interphase during Li+ intake/removal. The strategy is a simple and effective method which may be extended to other high-capacity anode materials with a large volume change and low electrical conductivity.