Thermal-conductivity and tensile-properties of BN, SiC and Ge nanotubes

Thermal-conductivity and axial tension of single-wall BN (Boron Nitride), SiC (Silicon Carbide) and Ge (Germanium) nanotubes are simulated through the MD (Molecular Dynamics) method, and the conductivity-temperature and loading-strain curves of the nanotubes are given. According to the obtained results, the differences of the nanotubes in thermal-conductivity and tensile-properties are investigated. The results show that the thermal-conductivity of all the tubes decreases with the increase of temperature and diameter; at the same temperature the BN tube has the best thermal-conductivity, whereas the SiC and Ge tubes have the comparable one; the Ge tube has both the worst anti-deformation and anti-loading capability, the BN tube the best anti-deformation one, and the BN and SiC tubes the comparable anti-loading one.     

Superelasticity of Carbon Nanocoils from Atomistic Quantum Simulations

A structural model of carbon nanocoils (CNCs) on the basis of carbon nanotubes (CNTs) was proposed. The Young’s moduli and spring constants of CNCs were computed and compared with those of CNTs. Upon elongation and compression, CNCs exhibit superelastic properties that are manifested by the nearly invariant average bond lengths and the large maximum elastic strain limit. Analysis of bond angle distributions shows that the three-dimensional spiral structures of CNCs mainly account for their unique superelasticity.     

铁掺杂TiO2纳米管阵列的制备及其电化学性能

在钛基体上,通过二次阳极氧化法和化学方法制备了不同Fe掺杂浓度的TiO2纳米管阵列,分析了所制备掺杂纳米管阵列的表面形貌、结构及化学组成,研究了其耐腐蚀性能。结果表明,0.05mol/L的Fe(NO3)3溶液浸泡可以不破坏纳米管阵列实现Fe离子高浓度掺杂。过高铁离子浓度溶液掺杂会损坏纳米管表面形貌,且实际Fe离子掺杂浓度低。经过低铁离子浓度溶液浸泡后,纳米管阵列表现出最好的耐腐蚀性。     

Electrospinning synthesis of novel lithium-rich 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 nanotube and its electrochemical performance as cathode of lithium-ion battery

In this study, a lithium-rich layered 0.4Li₂MnO₃·0.6LiNi_1/3Co_1/3Mn_1/3O₂ nanotube cathode synthesized by novel electrospinning is reported, and the effects of temperature on the electrochemical performance and morphologies are investigated. The crystal structure is characterized by X-ray diffraction patterns, and refined by two sets of diffraction data (R-3m and C2/m). Refined crystal structure is 0.4Li₂MnO₃·0.6LiNi_1/3Co_1/3Mn_1/3O₂ composite. The inductively coupled plasma optical emission spectrometer and thermogravimetric and differential scanning calorimetry analysis measurement supply reference to optimize the calcination temperature and heat-treatment time. The morphology is characterized by scanning and highresolution transmission electron microscope techniques, and the micro-nanostructured hollow tubes of Li-rich 0.4Li₂MnO₃·0.6LiNi_1/3Co_1/3Mn_1/3O₂ composite with outer diameter of 200-400 nm and the wall thickness of 50-80 nm are synthesized successfully. The electrochemical evaluation shows that 0.4Li₂MnO₃·0.6LiNi_1/3Co_1/3Mn_1/3O₂ sintered at 800 ℃ for 8 h delivers the highest capacity of the first discharge capacity of 267.7 mAh/g between 2.5 V and 4.8 V at 0.1C and remains 183.3 mAh/g after 50 cycles. The electrospinning method with heat-treatment to get micro-nanostructured lithium-rich cathode shows promising application in lithium-ion batteries with stable electrochemical performance and higher C-rate performance for its shorter Li ions transfer channels and stable designed structure.     

A novel approach to titania nanowire arrays as photoanodes of back-illuminated dye-sensitized solar cells

Titania nanowire arrays have been deposited on Ti foils through direct oxidizing the Ti substrate with aqueous hydrogen peroxide solutions containing melamine and nitric acid, and the applicability of such nanowire arrays to back-illuminated dye-sensitized solar cells studied in parallel with titania nanotube arrays on Ti foils. The low-temperature nitrogen adsorption measurement reveals that the film with nanowires 25 nm in diameter and 1 μm in length possesses a BET specific surface area of 59.0 m² g⁻¹, a value much larger than 26 m² g⁻¹ calculated for the nanotube with an inner diameter of 80 nm, an outer diameter of 120 nm and a total length of 3 μm. Assuming an indirect transition between band gaps, the nanowire film exhibits a bandgap of 3.1 eV, slightly larger than that of 3.0 eV for the nanotube one. A detailed electrochemical study suggests that, in comparison with the nanotube film, the nanowire one exhibits much lower saturated photocurrent and poorer conductivity under the Xe-lamp irradiation. However, when utilized to construct back-side illuminated dye-sensitized solar cells, the cell with the 2 μm-thick nanowire photoanode possesses significantly higher efficiency than the one with the 3 μm-thick nanotube photoanode. The relatively high energy conversion efficiency is contributed to the high specific surface area and the unique mesoporous structure of the titania nanowire arrays, which favors the adsorption of dye molecules.     

The Influence of Structural Defect on Mechanical Properties and Fracture Behaviors of Carbon Nanotubes

Due to the limitation of fabrication technologies nowadays, structural or atomistic defects are often perceived in carbon nanotubes (CNTs) during the manufacturing process. The main goal of the study aims at providing a systematic investigation of the effects of atomistic defects on the nanomechanical properties and fracture behaviors of single-walled CNTs (SWCNTs) using molecular dynamics (MD) simulation. Furthermore, the correlation between local stress distribution and fracture evolution is studied. Key parameters and factors under investigation include the number, type (namely the vacancy and Stone-Wales defects), location and distribution of defects. Results show that the nanomechanical properties of the CNTs, such as the elastic modulus, ultimate strength and ultimate strain, are greatly affected by the defects and also their percentage and type. It is also found that the CNTs present a brittle fracture as the strain attains a critical value, and in addition, the fracture crack tends to propagate along the high tensile stress concentration area. Moreover, the distribution pattern of defects is another driving factor affecting the nanomechanical properties of the CNTs and the associated fracture evolutions.     

Hydrogen generation under sunlight by self ordered TiO2 nanotube arrays

Highly ordered TiO₂ nanotube arrays generate a considerable interest for hydrogen generation by an electrochemical photocell, since ordered architecture of nanotube arrays provides a unidirectional electric channel for electron's transport. Here, we report the hydrogen generation by highly ordered TiO₂ nanotube arrays under actual sunlight in KOH electrolyte. The two-electrode electrochemical cell included an adjustable anode compartment capable of tracing the trajectory of the sun and a set of alkaline batteries connected with a rheostat for application of external bias. The results showed that the photocurrent responses of nanotube arrays match well with the intensity of solar irradiance on a clear summer day. Addition of ethylene glycol into KOH electrolyte as a hole scavenger enhanced the rate of hydrogen generation. A maximum photocurrent density of 31 mA/cm² was observed at 13:30 h, by focusing the sunlight with an intensity of 113 mW/cm² on the surface of the TiO₂ nanotube arrays in 1 M KOH electrolyte with 10 vol% ethylene glycol under an applied bias of 0.5 V. The observed hydrogen generation rate was 4.4 mL/h cm² under the focalized solar irradiance with an intensity between 104 mW/cm² and 115 mW/cm² from 10:00 to 14:20 h.     

Significantly enhanced charge conduction in electric double layer capacitors using carbon nanotube-grafted activated carbon electrodes

Carbon nanotube (CNT)-grafting by chemical vapor deposition was conducted to reduce the resistance of activated carbon fiber serving as an electrode for electric double layer capacitors. Sputtering deposition of Ni catalyst particles led to a uniform growth of CNTs on the carbon fiber surface through the tip-growth mechanism. Because sputtering deposition ensures little pore blockage (in comparison with wet-impregnation), the surface area decrease of the carbon fiber due to Ni loading was minimized. By using H₂SO₄ aqueous solution as the electrolyte, a capacitor cell assembled with the CNT-grafted fiber showed higher electron and electrolyte-ion conductivities relative to a cell assembled with the bare fiber. By increasing the discharging current density from 1 to 150 mA cm⁻², the bare fiber exhibited a capacitance loss of 17% while the CNT-grafted fiber showed a mitigated capacitance loss of only 7%. This developed CNT-grafting technique renders activated carbon fiber a promising electrode material for a variety of electrochemical applications.     

Initial stages of electrodeposition of metal nanowires in nanoporous templates

This paper describes the initial stages of electrochemical growth of Cu and Ni nanowires in polycarbonate (PC) track-etched membranes as the template. The diameters of the wires ranged from 50 to 200 nm. A thin Pt-Pd layer (∼30 nm thick) was sputter-deposited onto one side of the membrane and used as the cathode. The layer was not thick enough to seal the pore mouths. Cathodic current in the early stages of both Cu and Ni electrodeposition abruptly decreased after a period proportional to the pore diameter. Growing Cu grains plugged the pore openings causing the current to decrease, while the Ni deposition initially yielded a hollow tube in each pore resulting in a nanostructure transition of the tube to the wire at the growth front and a decrease in the current.     

Nanotube morphology changes for Ti-Zr alloys as Zr content increases

Nanotube morphology changes in Ti-Zr alloys as Zr content increases have been investigated. Ti-Zr (10, 20, 30 and 40 wt.%) alloys were prepared by arc melting and heat treated for 24 h at 1000 °C in an argon atmosphere. TiO₂ nanotubes were formed on the Ti-Zr alloys by anodization in H₃PO₄ containing 0.5 wt.% NaF. Electrochemical experiments were performed using a conventional three-electrode configuration with a platinum counter electrode and a saturated calomel reference electrode. Samples were embedded in epoxy resin, leaving an area of 10 mm² exposed to the electrolyte. Anodization was carried out using a scanning potentiostat, and all experiments were conducted at room temperature. Microstructures of the alloys were examined by optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). The Ti-Zr alloy microstructures observed by OM and FE-SEM changed from a lamellar structure to a needle-like structure with increasing Zr content. The microstructures also changed from β phase to increasing amounts of α phase as the Zr content increased. The number of large nanotubes formed by anodization decreased, and the number of small nanotubes increased, as the Zr content increased. The mean inner diameter ranged from approximately 150 to 200 nm with a tube-wall thickness of about 20 nm. The interspace between the nanotubes was approximately 60, 70, 100 and 130 nm for Zr contents of 10, 20, 30 and 40 wt.%, respectively.