250 µm long anodic TiO2 nanotubes with hexagonal self‐ordering

In this work we show that the growth of more than 250 µm thick self‐organized TiO₂ nanotube layers is possible, using an electrochemical approach in organic electrolytes. The tubes can grow as a hexagonal close packed pore array. Crucial parameters that decide on the dimensions are the fluoride ion concentration, the voltage and the anodization time. Self‐organized tube formation is restricted to a critical parameter range. Highest aspect ratio tubes can be achieved under a set of very optimized conditions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

Enhanced self‐ordering of anodic ZrO2 nanotubes in inorganic and organic electrolytes using two‐step anodization

This work reports on the optimized growth of self‐ordered ZrO₂ nanotubes in inorganic water‐based and organic electrolyte systems containing small amounts of fluoride employing a two‐step anodization process. We show how surface pretreatments of the metallic Zr substrate can drastically improve the growth and morphology of the resulting anodic ZrO₂ nanotube arrays. Using two step anodization and employing organic electrolytes, highly regular and ordered nanotubular ZrO₂ oxide layers can be grown to significantly increased tube lengths compared with aqueous electrolytes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A simple approach for synthesis of TiO2 nanotubes with through‐hole morphology

The present work reports a simple approach for fabrication of self‐standing titania (TiO₂) nanotube membranes with through‐hole morphology. The method is hydrofluoric acid free and the pore opening of TiO₂ nanotubes is performed by electrochemical thinning of the oxide barrier layer. A reduction of anodization voltage was applied at the end of the anodization process to cause a successful removal of the remaining barrier layer from the TiO₂ nanotubes during their detachment from the underlying titanium substrate. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

TiO2 nanotubes: photocatalyst for cancer cell killing

The present work reports on the use of self‐organized TiO₂ nanotube layers for the photocatalytic killing of cancer cells. TiO₂ nanotube layers with different dimensions (diameter 50 nm and 100 nm, thickness 800 nm and 1.3 μm, respectively) were grown by anodization of Ti. Upon low dose of UV irradiation, the vitality of cancer cells cultured on these nanotube layers was significantly affected – the cells reduced their shape and size and a significant amount of the dead cells was found. These results demonstrate that self‐organized TiO₂ nanotube layers can be used for photo‐induced cancer cell killing. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Efficient solar energy conversion using TiO2 nanotubes produced by rapid breakdown anodization – a comparison

We report on dye‐sensitization of different TiO₂ nanotube layers, their photoelectrochemical response and their efficiency for solar energy conversion. The tubes compared in this study were either grown by controlled Ti anodization in fluoride containing electrolytes or by rapid breakdown anodization (RBA) of Ti in fluoride free electrolytes. After converting the different tube layers to anatase and sensitizing with Ru‐dye (N719), clearly layers consisting of RBA‐NTs show a significantly higher photoresponse and conversion efficiencies than tubes formed under self‐ordering conditions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of Y-junction carbon nanotubes by catalytic CVD of methane

Y-junction carbon nanotubes were grown by catalytic CVD of methane at 700 ^°C on NiO-CuO-MoO(7:2:1) (w/w/w)/SiO₂ catalyst. For comparison, NiO-CuO(8:2) (w/w)/SiO₂ and NiO-MoO(8:2) (w/w)/SiO₂ catalysts were tested for carbon nanotube formation. TEM analysis indicates that no Y-junction structures were formed with the latter two catalysts. This finding elucidates why the addition of a small amount of MoO to NiO-CuO/SiO₂ catalyst is crucial for enhancing the formation of Y-junction carbon nanotubes.     

Anatase type titania nanotube arrays direct fabricated by anodization without annealing

In this paper, anatase type titania nanotube arrays were direct fabricated by anodization in dimethyl sulfoxide electrolyte containing 1 wt% HF solution at above 50 °C without subsequently annealing. The length of the nanotubes decreases with increasing anodization temperature from about approximately 15 μm at 40 °C to approximately 4.5 μm at 60 °C. High resolution transmission electron microscope images and selected area electron diffraction pattern confirm the polycrystalline anatase specimen consisting of many nanocrystals with a random orientation.     

Amine-functionalized boron nitride nanotubes

The surface of boron nitride nanotubes (BNNTs) has been functionalized with amine groups via ammonia plasma irradiation. The functionalized tubes were characterized by Fourier transform infrared spectroscopy and electron energy loss spectroscopy. Amine-functionalized BNNTs were found to be highly dispersible in chloroform, and are predicted to form the basis of a new class of chemically reactive nanostructures.     

Free standing double walled boron nanotubes

Based on density functional calculations we propose stable structures of free standing double walled boron nanotubes in the form of two single walled boron nanotubes (SWBNTs) inside one another. Puckering of the boron sheets allows the inner atoms of the outer wall and outer atoms of the inner wall to be matched giving sp-type hybrid σ bonding between the walls. The structural stability, in the case of double walled tubes, increases as the bond interaction between the walls strengthens. All the optimized structures reported in this study are electronically conducting in good agreement with the previously calculated metallic behavior of the experimentally observed SWBNTs.     

Hot filament effects on CVD of carbon nanotubes

We grew vertically aligned CNTs via HFCVD using mixtures of methane and hydrogen as feedstock, and investigated the dependence of CNT growth on feedstock composition, filament temperature, and filament types. At the filament temperature of 2050 °C tungsten filaments were more efficient for CNT growth than tantalum ones, and higher CNT growth rates were observed when tungsten filaments were operated at 1900 °C. Regardless of filament temperatures and types, monotonic increase in growth rate of vertically aligned CNTs was observed as we increased the methane concentration in the feedstock. In‐situ investigation of feedstock dissociation revealed the generation of various radical species, and, moreover, a strong correlation between CNT growth rates and relative mole fractions of single‐carbon radicals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonlinear Deformation Processes and Damage Modes of Super Carbon Nanotubes with Armchair-Armchair Topology

The tensile deformations and fractures of super carbon nanotubes （SCNTs） with armchair-armchair topology are investigated by using the atomic-scale finite element method. SCNTs generated from carbon nanotubes （CNTs） with different characteristic aspect ratios are found to have different nonlinear behaviours under uniaxial tensions. Specifically, an SCNT wi~h higher aspect ratio has three distinct stages： rotation, stretch and rupture, while an SCNT with lower aspect ratio has only two stages. This information may compensate for previous work and enrich our knowledge about Y-branched CNTs and SCNTs.     

ZnSe‐based conductive nanotemplates for nanofabrication

We show that anodic etching of n‐type ZnSe crystalline substrates leads to the formation of pores which, after nucleation at surface defects, prove to follow the current lines, exhibiting multiplication until the front of the porous network covers the whole available space. No growth of crystallographically oriented pores has been observed in ZnSe. The formation of multilayer porous structures is realized, including layers subjected to successive porosification at two different length scales. The electrochemical pulsed deposition of arrays of Pt nanotubes in the porous ZnSe matrix is demonstrated. The obtained results show that porous ZnSe structures are promising for use as conductive and optically transparent nanotemplates for nanofabrication, in particular for the important application of metal nanotubes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of doping on electronic properties of double-walled carbon and boron nitride hetero-nanotubes

The effect of boron nitride (BN) doping on electronic properties of armchair double-walled carbon and hetero-nanotubes is studied using ab initio molecular dynamics method. The armchair double-walled hetero-nanotubes are predicted to be semiconductor and their electronic structures depend strongly on the electronic properties of the single-walled carbon nanotube. It is found that electronic structures of BN-doped double-walled hetero-nanotubes are intermediate between those of double-walled boron nitride nanotubes and double-walled carbon and boron nitride hetero-nanotubes. Increasing the amount of doping leads to a stronger intertube interaction and also increases the energy gap.     

Electrocatalytic oxidation of formaldehyde on platinum well-dispersed into single-wall carbon nanotube/polyaniline composite film

Single-walled carbon nanotubes (SWNTs)/polyaniline (PANI) composite films with good uniformity and dispersion were prepared by electrochemical polymerization of aniline containing well-dissolved SWNTs. The composite films were dispersed Pt by electrodeposition technique. The presence of SWNTs and platinum in the composite film was confirmed by XRD analysis and scanning electron microscopy (SEM). Four-point probe investigation and electrochemical impedance spectroscopy (EIS) revealed that the well arrangement of PANI coated SWNTs in these films enhanced electric conductivity and facilitated the charge-transfer of the composite films. Cyclic voltammogram (CV) and chronoamperogram showed that Pt-modified SWNT/PANI composite film performs higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formaldehyde oxidation. The results imply that the SWNT/PANI composite film as a promising support material improves the electrocatalytic activity for formaldehyde oxidation greatly.     

Functional multi-walled carbon nanotube/polyaniline composite films as supports of platinum for formic acid electrooxidation

Embedding of carbon nanotubes in conducting polymeric matrices for various nanocomposites material is now a popular area. In this article, a concise chemical method has been described for the preparation of homogeneous nanocomposite of multi-walled carbon nanotube (MWNT)/polyaniline (PANI) by electrochemical codeposition. For this we functionalized the MWNTs via the diazotization reaction. This helped to disperse the nanotubes in aniline. The composite films were dispersed Pt by electrodeposition technique. The presence of MWNTs and platinum in the composite films was confirmed by XRD analysis and transmission electron microscopy (TEM). Four-point probe investigations revealed that the MWNT/PANI composite films exhibited a good conductivity. Cyclic voltammograms (CV) showed that Pt-modified MWNT/PANI composite films perform higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formic acid oxidation. The results imply that the MWNT/PANI composite films as a promising support material improves the electrocatalytic activity for formic acid oxidation greatly.     

Adsorption and electrochemistry of hemoglobin on Chi-carbon nanotubes composite film

This study describes the noncovalently functionalization of carbon nanotubes (CNTs) with natural biopolymer chitosan (Chi) as substrate for hemoglobin (Hb) immobilization. The noncovalently Chi-functionalized CNTs possessed an improved solubility in aqueous solution and was beneficial to form three-dimensional configuration of the CNTs film electrode. The adsorbed Hb in Chi-CNTs interface showed a pair of quasi-reversible redox peak with a formal potential of −0.34 V (vs. SCE) in 0.10 M pH 7.0 phosphate buffer solution and possessed good bioelectrochemical catalytic activities toward the reductions of H₂O₂.     

Highly selective growth of vertically aligned double‐walled carbon nanotubes by a controlled heating method and their electric double‐layer capacitor properties

Vertically aligned double‐walled carbon nanotubes (DWCNTs) with the highest selectivity of 90% were synthesized by a controlled heating method and their electric double‐layer capacitor characteristics were evaluated. DWCNT arrays had a specific capacitance of 83 F/g, which is one of the highest values among CNT arrays in a nonaqueous solution and is almost equivalent to that for single‐walled CNT (SWCNT) arrays reported previously. At the same specific capacitance, DWCNTs with superior structural properties are more promising for practical capacitors than SWCNTs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and Raman scattering study of double-walled carbon nanotube peapods

Double-walled carbon nanotubes (DWNTs) encapsulating C₆₀ fullerenes were successfully synthesized by gas phase diffusion method. The obtained peapods were examined using high-resolution transmission electron microscopy (HRTEM). The HRTEM images indicate that the ordered packing phases of fullerene molecules inside are sensitively related to the inner-tube radius of DWNTs. Also, Raman measurements were carried out for the first time to characterize DWNTs peapods. There are obvious differences between the Raman spectrum of DWNTs peapods and that of SWNTs peapods. The intensities of resonances from C₆₀ in the former are much stronger than those in the latter. In addition, changes of tangential mode (TM) and radial breathing mode (RBM) of DWNTs after C₆₀ doping were observed. The possible reasons are discussed in the text.     

Theoretical study of the adsorption of CO2 on tungsten carbide nanotubes

The adsorption of CO₂ on the single-walled tungsten carbide nanotubes has been investigated employing density functional theory method. The center of a hexagon of tungsten and carbon atoms in sites on tungsten carbide nanotube surfaces is the most stable adsorption site for CO₂ molecule, with a binding energy of −1.68 eV (−38.72 kcal/mol) and a WO binding distance of 1.95 Å. Furthermore, the adsorption of CO₂ on the single-walled carbon nanotubes has been investigated. Our first-principles calculations predict that the CO₂ adsorptive capacity of tungsten carbide nanotubes is about quadruple that of carbon nanotubes. This might have potential for greenhouse gas detection and bioremediation.     

Cutting of multi walled carbon nanotubes

Multi walled carbon nanotubes (MCNT) synthesized by CVD method have been successfully cut into different lengths by controlling H₂SO₄/HNO₃ (5:3) oxidation time. During the cutting process H₂SO₄ and HNO₃ were added independently and the oxidation processes were carried out at a lower temperature to void excess weight loss and damage to MCNT. The resulting shorted MCNT (s-MCNT) formed stable dispersion state in the polar solvents without the help of surfactants that provided possibility for further functionalization and application. Moreover, NaOH solution was used to determine the total percentage of acidic sites and the total percentage of acidic sites are about 0.2-1%.