Wuzong Zhou

Trititanate Nanotubes Made via a Single Alkali Treatment

A new type of crystalline trititanate nanotube (see Figure and inside front cover for a 3D drawing) has been synthesized in a single alkali treatment. Novel formation mechanisms of the nanotubes are proposed based on a series of experiments. In the process, NaOH can be regarded as a catalyst and a reusable source. The synthetic method is extremely inexpensive and may be applied to synthesize other oxide nanostructures.

Disruption of extended defects in solid oxide fuel cell anodes for methane oxidation

Point defects largely govern the electrochemical properties of oxides: at low defect concentrations, conductivity increases with concentration; however, at higher concentrations, defect–defect interactions start to dominate. Thus, in searching for electrochemically active materials for fuel cell anodes, high defect concentration is generally avoided. Here we describe an oxide anode formed from lanthanum-substituted strontium titanate (La-SrTiO3) in which we control the oxygen stoichiometry in order to break down the extended defect intergrowth regions and create phases with considerable disordered oxygen defects. We substitute Ti in these phases with Ga and Mn to induce redox activity and allow more flexible coordination. The material demonstrates impressive fuel cell performance using wet hydrogen at 950 °C. It is also important for fuel cell technology to achieve efficient electrode operation with different hydrocarbon fuels, although such fuels are more demanding than pure hydrogen. The best anode materials to date—Ni-YSZ (yttria-stabilized zirconia) cermets—suffer some disadvantages related to low tolerance to sulphur, carbon build-up when using hydrocarbon fuels (though device modifications and lower temperature operation can avoid this) and volume instability on redox cycling. Our anode material is very active for methane oxidation at high temperatures, with open circuit voltages in excess of 1.2 V. The materials design concept that we use here could lead to devices that enable more-efficient energy extraction from fossil fuels and carbon-neutral fuels.

Formation Mechanism of CaTiO3 Hollow Crystals with Different Microstructures

The crystal growth of CaTiO(3) hollow crystals with different microstructures has been investigated. In a water-free poly(ethylene glycol) 200 (PEG-200) solution, CaTiO(3) nanocubes formed first. The nanocubes underwent an oriented self-assembly into spherical particles, enhanced by the surface-adsorbed polymer molecules. Since the growth of nanocubes and their aggregation took place simultaneously, the nanocubes in the outer shells were larger than those in the cores. Disappearance of the small nanocubes in the cores of the spheres during an Ostwald ripening process led to spherical hollow crystals. Addition of a small amount of water (1.25 vol %) in the polymer solution enhanced surface recrystallization of the aggregated spheres, forming a cubic morphology. The orthorhombic distortion of the perovskite CaTiO(3) structure did not have a significant effect on the nanocube aggregation, resulting in a domain structure in the shells. Single-crystalline hollow cubes were produced with a slightly higher water content, e.g., 5 vol %. This process of (1) aggregation of nanocubes and (2) surface crystallization followed by (3) surface-to-core extension of recrystallization gives a good example of the reversed crystal growth route in ceramic materials. The proposed formation mechanism of the hollow CaTiO(3) crystals would enable us to control the microstructures of these materials and to explain the formation of many other hollow crystals.

Polymerized carbon nanobells and their field-emission properties

Aligned nitrogen-containing carbon nanofibers consisting of polymerized “nanobells” have been grown on a large scale using microwave plasma-assisted chemical-vapor deposition with a mixture of methane and nitrogen. A greater part of the fiber surface consists of open ends of the graphitic sheets. A side-emission mechanism is proposed. A low-threshold field of 1.0 V/μm and a high-emission current density of 200 mA/cm2 for an applied field of 5–6 V/μm were achieved, implying that the materials have a high potential for future application as electron field emitters, especially in flat-panel displays.

Selective Co-catalysed growth of novel MgO fishbone fractal nanostructures

Abstract Novel MgO fishbone- or fern-like nanostructures, and Al 2 O 3 fibre networks, have been generated for the first time from an Al 2 O 3 matrix containing MgO and SiO 2 . The significance of the selective Co-catalysed growth of the elegant three-dimensional fractal fishbones is discussed. We have adopted the vapour–liquid–solid (V–L–S) and nucleation–aggregation (N–A) mechanisms, which take effect equally during the heating process, to account for the formation of these uniquely stacked fractal structures.

Formation, morphology control and applications of anodic TiO2 nanotube arrays

Anodic titanium dioxide films, especially anodic TiO2 nanotube arrays, have attracted extensive interest in the past decade. A number of electrolytes, either aqueous or non-aqueous, fluoride containing or fluoride free, have been chosen to grow anodic titanium oxide films. With great improvements in the morphology control on porosity, pore size, nanotube length and pore ordering, anodic titanium oxide films have been widely applied in photochemical water splitting, hydrogen sensing, dye-sensitized solar cells, templating for low dimensional nanomaterials and biomedical research. This article presents a brief review of the progress to date in the formation mechanism, morphology control and some applications of these smart materials.

Effect of structural aluminium on the mesoporous structure of MCM-41

X-ray diffraction (XRD), 27Al magic-angle-spinning (MAS) NMR, N2 adsorption measurements and transmission electron microscopy (TEM) show that the one-dimensional channels of the mesoporous aluminosilicate MCM-41 containing only structural (four-coordinate) aluminium are much shorter in the sample with an Si/Al ratio of 34.1 (ca. 160 A) than in purely siliceous MCM-41 (ca. 400 A). The quality of the XRD patterns rapidly deteriorates as the aluminium content of the solid increases. However, aluminosilicate MCM-41 still preserves a perfect mesoporous structure with an average pore diameter of ca. 30 A, a surface area of 990 m2 g–1 and a pore volume of 0.74 cm3 g–1. When the Na+ form of aluminosilicate MCM-41 is transformed into the Bronsted acidic H+ form, and some of the aluminium is removed from the structure, the uniform hexagonal mesopores partially collapse and macropores 200–2000 A in diameter are formed.

Zeolite GdNaY Nanoparticles with Very High Relaxivity for Application as Contrast Agents in Magnetic Resonance Imaging

In this paper we explore Gd(3+)-doped zeolite NaY nanoparticles for their potential application as a contrast agent in magnetic resonance imaging (MRI). The nanoparticles have an average size of 80-100 nm, as determined by TEM and XRD. A powdered sample loaded with La3+ was characterised by means of multinuclear solid-state NMR spectroscopy. The NMR dispersion (NMRD) profiles obtained from aqueous suspensions of samples with Gd3+ doping ratios of 1.3-5.4 wt% were obtaining at different temperatures. The relaxivity increases drastically as the Gd3+ loading decreases, with values ranging between 11.4 and 37.7 s-1 mM-1 at 60 MHz and 37 degrees C. EPR spectra of aqueous suspensions of the samples suggest that an interaction between neighbouring Gd3+ ions within the same particle produces a significant increase in the transversal electronic relaxation rates in samples with a high Gd3+ content. The experimental NMRD and EPR data are explained with the use of a model that considers the system as a concentrated aqueous solution of Gd3+ in the interior of the zeolite that is in exchange with the bulk water outside the zeolite. The results obtained indicate that the Gd3+ ion is immobilised in the interior of the zeolite and that the relaxivity is mainly limited by the relatively slow diffusion of water protons from the pores of the zeolite channels into the bulk water.

Crystalline WO3 nanowires synthesized by templating method

Abstract A new method is developed to obtain crystalline nanowires of WO 3 using mesoporous silica SBA-15 as template. The method includes aminosilylation of the surface silanols within SBA-15 channels, anchoring of the heteropoly acid (HPA) to the grafted amine groups, thermal decomposition of the HPA, and removal of the silica framework with HF. The formation of the crystalline nanowires is monitored by the in situ XRD technique. TEM images intuitively confirm that the nanowires are uniform in diameter and HRTEM images further indicate that each nanowire belongs to single crystal although the growth orientations of these nanowires are different.

Synthesis and field-emission behavior of highly oriented boron carbonitride nanofibers

Large-area highly oriented boron carbonitride (BCN) nanofibers with various compositions were synthesized directly on polished polycrystalline nickel substrates from a gas mixture of N2, H2, CH4, and B2H6 by bias-assisted hot-filament chemical-vapor deposition. The morphology of BCN nanofibers was examined by scanning electron microscopy, the nanofiber structure was studied by high-resolution transmission electron microscopy, and the chemical composition of individual nanofibers was determined by electron energy-loss spectroscopy. Field-emission behavior of the BCN nanofibers was characterized and a high emission current density of about 20–80 mA/cm2 at a low electric field of 5–6 V/μm implies a promising application as field-emission sources.