Cheng-Wei Wang

Very long carbon nanotubes

Carbon nanotubes can now be produced in large quantities by either arc methods, or thermal decomposition of hydrocarbons,. Here we report that pyrolysis of acetylene over iron/silica substrates is an effective method with which to produce very long, multiwalled carbon nanotubes that reach about 2 mm in length, which is an order of magnitude longer than that described in most previous reports,.

Enhanced field emission from hydrogenated TiO2 nanotube arrays

The field emission (FE) properties of TiO(2) nanotube arrays (TNAs) synthesized by anodization are dramatically improved after hydrogenation at various temperatures in a range of 400-550 °C. Compared with pristine TNAs, the turn-on fields of hydrogenated TNAs (H:TNAs) are significantly decreased from 18.23 to 1.75 V μm(-1), and closely related to hydrogenation temperature. Importantly, the optimized sample of H:TNAs prepared at 550 °C shows excellent FE performances involving both a low turn-on field of 1.75 V μm(-1), a high current density of 4.0 mA cm(-2) at 4.50V μm(-1), and a remarkable FE stability over 480 min. The substantially enhanced FE properties can be attributed to the combination of a typical tubular morphology, a reduced work function and the improved conductivity of H:TNAs.

Highly Flexible Coaxial Nanohybrids Made from Porous TiO2 Nanotubes

Anatase TiO(2), an n-type semiconductor, has gained considerable research interest over several decades due to its photocatalytic activity. Most recently, its properties for photoelectrical conversion in solar cells has been explored. Anodized TiO(2) nanotube (NT) arrays have been developed and possess improved photocatalytic, sensing, photoelectrolystic, and photovoltaic properties. The present work describes using TiO(2) as the building block to form ordered heterojunctions via simple electrodeposition with materials of potential interest, including conducting polymers (polypyrrole, poly(3-hexylthiophene)), inorganic semiconducting materials (CdS), and metals (Ni and Au, etc.). A key finding is that the synthesized TiO(2) NT-nanowires(nanotubes) nanohybrids are highly flexible after being peeled off from mother substrates, which is in contrast to more fragile pure TiO(2) NTs. These highly flexible coaxial nanohybrids are expected to have potent applications.

Enhancement of the Field Emission from the TiO2 Nanotube Arrays by Reducing in a NaBH4 Solution

A mass of oxygen vacancies are successfully introduced into TiO2 nanotube arrays using low-cost NaBH4 as a reductant in a liquid-phase environment. By controlling and adjusting the reduction time over the range of 0-24 h, the doping concentration of the oxygen vacancy is controllable and eventually reaches saturation. Meanwhile, the thermal stability of oxygen vacancies is also investigated, indicating that part of the oxygen vacancies remain stable up to 250 °C. In addition, this liquid-phase reduction strategy significantly lowers the requirements of instruments and cost. More interesting, reduced TiO2 nanotube arrays show drastically enhanced field emission performances including substantially decreased turn-on field from 25.01 to 2.65 V/μm, a high current density of 3.5 mA/cm(2) at 7.2 V/μm, and an excellent field emission stability and repeatability. These results are attributed to the oxygen vacancies obtained by reducing in NaBH4 solution, resulting in a reduced effective work function and an increased conductivity.

Low-temperature liquid phase reduced TiO2 nanotube arrays: synergy of morphology manipulation and oxygen vacancy doping for enhancement of field emission.

The partially reduced TiO(2) nanotube arrays (TNAs) are prepared via an uncomplicated and low-cost liquid phase reduction strategy using NaBH(4) as the reducing agent. By controlling and adjusting the reduction temperatures from 30 to 90 °C, the reduction treatment can not only change their surface morphology but also introduce oxygen vacancies into them, resulting in an optimized morphology, elevated Fermi-level, reduced effective work function and improved conductivity of the TNAs. Meanwhile, the thermal and long-term stability of oxygen vacancy are also investigated, indicating that the oxygen vacancies retain long-term stability from room temperature up to 150 °C. More interesting, partially reduced TNAs show drastically enhanced field emission (FE) performances including substantially decreased turn-on field from 18.86 to 1.53 V μm(-1), a high current density of 4.00 mA cm(-2) at 4.52 V μm(-1), and an excellent FE stability and repeatability. These very promising results are attributed to the combination of the optimized morphology and introduced oxygen vacancies, which can increase FE sites, reduce effective work function and increase conductivity.

Photoluminescence properties of porous anodic aluminium oxide membranes formed in mixture of sulfuric and oxalic acid

Porous anodic aluminium oxide (AAO) membranes with ordered nanopore arrays were fabricated electrochemically in the mixture electrolytes with various volume ratios of 0.3 M H2SO4 and 0.3 M C2H2O4. Photoluminescence (PL) properties of as-prepared AAO membranes were investigated. It was found that a 290 nm emission was observed in the AAO samples formed in the H2SO4 electrolyte and a 325 nm emission in the samples formed in the C2H2O4 electrolyte, respectively. For the AAO membranes anodized in the mixture electrolytes, both 290 and 325 nm emissions appeared together; moreover, the 290 nm peak was weakened, and the 325 nm peak was enhanced dramatically. It is suggested that the 290 and 325 nm emissions could be attributed to the luminescent centres transformed from sulfuric and oxalic impurities, respectively, and that nonradiative energy transfer occurs between these two kinds of PL centres.

Reduction of Friction of Metals Using Laser-Induced Periodic Surface Nanostructures

We report on the effect of femtosecond-laser-induced periodic surface structures (LIPSS) on the tribological properties of stainless steel. Uniform periodic nanostructures were produced on AISI 304L (American Iron and Steel Institute steel grade) steel surfaces using an 800-nm femtosecond laser. The spatial periods of LIPSS measured by field emission scanning electron microscopy ranged from 530 to 570 nm. The tribological properties of smooth and textured surfaces with periodic nanostructures were investigated using reciprocating ball-on-flat tests against AISI 440C balls under both dry and starved oil lubricated conditions. The friction coefficient of LIPSS covered surfaces has shown a lower value than that of the smooth surface. The induced periodic nanostructures demonstrated marked potential for reducing the friction coefficient compared with the smooth surface.

Enhanced field emission from hydrogenated SnO2 nanoparticles embedded in TiO2 film on fluorinated tin oxide substrate

Hydrogenated SnO2 nanoparticles (H:SNPs) were prepared on fluorinated tin oxide covered glass using reduction and hydrogenation technologies with TiO2 sol. By adjusting the hydrogenated temperature over the range of 400–550 °C, the H:SNPs were uniformly embedded in the TiO2 film, exhibiting the ability to precisely control their density and size using this method. Simultaneously, their band structures were modified, resulting in a reduced work function and an increased electrical conductivity. Hence, the optimized H:SNPs prepared at 500 °C showed excellent field emission (FE) performances, with both a low turn-on field of 3.81 V/μm and remarkable FE stability over a 480-min period.

Field emission property of carbon-doped TiO2 nanotube arrays with controllable doping content of carbon

To achieve an optimum cold cathode emitter, well-aligned TiO2/Ti nanotube arrays (TNAs) were synthesized by anodic oxidation and doped with carbon by pyrolysis of C2H2 at 550 °C. By controlling the carbon doping content, the field emission (FE) properties of carbon-doped TiO2/Ti nanotube arrays (C-TNAs) were optimized. Compared with the high turn-on field of 19.19 V/μm from pure TNAs, the turn-on field of C-TNAs was decreased to 11.60, 6.35, 4.10, and 5.77 V/μm when the doping content of carbon was increased to 0.62, 0.82, 1.81, and 3.31 at. %, respectively. Obviously, the FE properties of TNAs were dramatically improved and optimized by adjusting the carbon doping content, which can be attributed to the typical tubular morphology, an enhanced conductivity, and a reduced work function.

Communication: Non-monotonic evolution of dynamical heterogeneity in unfreezing process of metallic glasses

The relaxation dynamics in unfreezing process of metallic glasses is investigated by the activation-relaxation technique. A non-monotonic dynamical microstructural heterogeneities evolution with temperature is discovered, which confirms and supplies more features to flow units concept of glasses. A flow unit perspective is proposed to microscopically describe this non-monotonic evolution of the dynamical heterogeneities as well as its relationship with the deformation mode development of metallic glasses.