Fabrication of micrometer-scale spherical titanate nanotube assemblies with high specific surface area

Three-dimensional (3D) micrometer-scale spherical titanate nanotube assemblies have been successfully synthesized via hydrothermal synthesis from Ti powders. The obtained titanate nanotube assemblies have a diameter of around 4-7 µm. The titanate nanotubes have an outer diameter of around 9-10 nm, an inner diameter around 3-4 nm and a length of several micrometers. A proposed two-stage growth mechanism was applied to explain the formation process of these titanate nanotube assemblies. We have also demonstrated that these titanate assemblies have higher specific surface area and larger adsorption capacity than that of titanate nanotubes. And they could possibly be further used in photocatalysts, separation technologies, energy-storage technologies and so on.     

Adsorptive separation and photocatalytic degradation of methylene blue dye on titanate nanotube powders prepared by hydrothermal process using metal Ti particles as a precursor

Titanate nanotube powders (TNTPs) with the twofold removal ability, i.e. adsorptive separation and photocatalytic degradation, are synthesized under hydrothermal conditions using metal Ti particles as a precursor in the concentrated alkaline solution, and their morphology, structure, adsorptive and photocatalytic properties are investigated. Under hydrothermal conditions, the titanate nanotubes (TNTs) with pore diameter of 3-4 nm are produced on the surface of metal Ti particles, and stacked together to form three-dimensional (3D) network with porous structure. The TNTPs synthesized in the autoclave at 130 °C for 24 h exhibits a maximum adsorption capability of about 197 mg g⁻¹ in the neutral methylene blue (MB) solution (40 mg L⁻¹) within 90 min, the adsorption process can be described by pseudo second-order kinetics model. Especially, in comparison with the adsorptive and the photocatalytic processes are performed in turn, about 50 min can be saved through synchronously utilizing the double removal ability of TNTPs when the removal ratio of MB approaches 95% in MB solution (40 mg L⁻¹) at a solid-liquid (S/L) ratio of 1:8 under ultraviolet (UV) light irradiation. These 3D TNTPs with the twofold removal properties and easier separation ability for recycling use show promising prospect for the treatment of dye pollutants from wastewaters in future industrial application.     

Nanocrystallized steel surface by micro-shot peening for catalyst-free carbon nanotube growth

Nanocrystallized steel surface by micro-shot peening (MSP) were applied to carbon nanotube growth in this study. Micro-shot peening treatment severely deformed steel surface and nanocrystallized surface layer was formed by the plastic deformation. The grain sizes of the nanocrystallized layer were 10-30 nm after 300 s of MSP treatment. On the nanocrystallized surface, carbon nanotubes were formed with thermal chemical vapour deposition without catalysts. Before carbon nanotube growth, the nanocrystallized steel surface was reduced with H₂/N₂ gas at 600 °C. The carbon nanotube growth was performed at 600 °C with C₂H₂ gas carried by H₂/N₂ gas. The carbon nanotubes formed on the nano-structured surface was multi-walled carbon nanotube and the diameter was 10-20 nm. The reduction process before carbon nanotube growth was essential to form carbon nanotubes on the nanocrystallized surface with MSP.     

Titania Nanotubes Prepared by Chemical Processing

We report a new method for the synthesis of titanium oxide (TiO₂) nanotubes. When anatase-phase- or rutile-phase-containing TiO₂ was treated with an aqueous solution of 5–10 M NaOH for 20 h at 110 °C and then with HCl aqueous solution and distilled water, needle-shaped TiO₂ products were obtained (diameter ≈ 8 nm, length ≈ 100 nm). The needle-shaped products are nanotubes with inner diameters of approximately 5 nm and outer diameters of approximately 8 nm. The formation mechanism of titania nanotubes is discussed in terms of the detailed observation of the products by transmission electron microscopy: the crystalline raw material is first converted to an amorphous product through alkali treatment, and subsequently, titania nanotubes are formed after treatment with distilled water and HCl aqueous solution.     

Synthesis of carbon nanotubes at low temperature by filament assisted atmospheric CVD and their field emission characteristics

Multiwalled carbon nanotubes (MWNTs) were synthesized using a hot filament assisted chemical vapor deposition (CVD) at the atmospheric pressure at a substrate temperature of 550 °C. The size of nanotubes was controlled by changing the size of catalyst particles. The structure and composition of these nanotubes were investigated using scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The electron field emission current of MWNTs was also measured. It was found that the nanotubes with smaller the diameter had higher the emission current levels though synthesis conditions except catalyst particles were the same. These as-grown MWNTs had emission current densities of 6.5 mA/cm² and 2.5 mA/cm² at 1 V/μm for 5-8 nm and 20 nm size carbon nanotube samples, respectively. The results indicated that the MWNTs synthesized had low emission threshold voltages and high emission current levels that are favorable properties for field emission-based display device applications.     

Cathodoluminescence of TiO2 nanotubes prepared by low-temperature anodization of Ti foils

We show that anodization of Ti sheets in an ethylene glycol and HF containing electrolyte at temperatures under 0 °C results in the formation of a self-arranged ordered porous structure at the top surface of the sample. This perforated surface structure initiates the growth of an ordered array of titania nanotubes. The inner diameter of nanotubes can be modified in a controlled fashion in the range from 10 nm to more than 250 nm through the change of the electrolyte temperature from −20 °C to + 50 °C. The spectral distribution of cathodoluminescence from a cluster of nanotubes clearly demonstrates the formation of resonator modes which are separated from each other by around 200 meV.     

Preparation and characterization of titanate nanotubes/carbon composites

Titanate nanotubes/carbon composites(TNT/CCs) were synthesized by allowing carbon-coated TiO₂ (CCT) powder to react with a dense aqueous solution of NaOH at 120 °C for a proper period of time. As-prepared CCT and TNT/CCs were characterized by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectrometry. The processes for formation of titanate nanotubes/carbon composites were discussed. It was found that the TiO₂ particles in TiO₂-carbon composite were enwrapped by a fine layer of carbon with a thickness of about 4 nm. This carbon layer functioned to inhibit the transformation from anatase TiO₂ to orthorhombic titanate. As a result, the anatase TiO₂ in CCT was incompletely transformed into orthorhombic titanate nanotubes upon 24 h of reaction in the dense and hot NaOH solution. When the carbon layers were gradually peeled off along with the formation of more orthorhombic titanate nanotubes at extended reaction durations (e.g., 72 h), anatase TiO₂ particles in CCT were completely transformed into orthorhombic titanate nanotubes, yielding TNT/CCs whose morphology was highly dependent on the reaction time and temperature.     

In-situ growth of titania nanoparticles in electrospun polymer nanofibers at low temperature

In-situ growth of titania nanoparticles in poly (ethylene terephthalate) (PET) nanofibers has been successfully achieved by combining sol-gel method and electrospinning process. Titania precursor, tetra-n-butyl titanate (TBT), was firstly hydrolyzed in trifluoroacetic acid (TFA), and then blend with a solution of PET in mixture of trifluoroacetic acid/dichloromethane (TFA/DCM) to form a homogeneous solution for electrospinning. Titania nanoparticles in-situ generated in the electrospun nanofibers via a hydrothermal treatment process at 70 °C-90 °C. The morphology and crystallinity of PET/TiO₂ hybrid nanofibers were investigated using TEM and DSC. The results showed that titania nanoparticles of anatase phase with an average diameter of about 10 nm in-situ generated both inside and on the surface of PET electrospun nanofibers. The reversible networks formed between titania nanoparticles and PET macromolecular chains led to considerable decrease of PET crystallinity.     

Fabrication of carbon nanotubes by electrical breakdown of carbon-coated Au nanowires

A carbon nanotube has been generated by the electrically-induced breakdown of a carbon-coated Au nanowire. Under high current density the Au in the nanowire migrates towards both the anode and cathode resulting in a free-standing carbon nanotube and a 73% reduction in resistance. The resistivity of the carbon nanotube was < 8 × 10^− 5 Ω m and it could cope with a current density > 1.8 × 10¹¹ A/m², indicating a structural change from amorphous to graphitic carbon. The dimensions of carbon nanotubes produced in this way have an internal diameter controlled by the parent metal nanowire template.     

Synthesis, electrical and magnetic characterization of core-shell silicon carbo-nitride coated carbon nanotubes

We demonstrate synthesis, electrical and magnetic characterization of silicon carbo-nitride (SiCN) coated multiwalled carbon nanotubes in a core-shell structure. The core formed by a carbon nanotube had a diameter in the range of 10-100 nm. The shell was synthesized by pyrolysis of an SiCN precursor on the surface of carbon nanotubes. Electrical resistivity of an individual composite nanotube was measured to be ~ 2.55 × 10³ Ω cm. The magnetic measurements performed by a superconducting quantum interference device on the composite nanotubes in the temperature range of 5-300 K show a reduced coercive field with increasing temperatures. The monolayer thick coating of an ultra high temperature multifunctional ceramic SiCN makes these composite nanotubes very promising for sensing applications in harsh environments.     

Kinetics of titania nanotube formation by anodization of titanium films

We report the kinetics of titania nanotube length evolution during anodization of titanium films. Our results show that the nanotube length increase is thermally activated, and governed by voltage-dependent activation energy 0.6 eV ≤ E^eff ≤ 1.1 eV expressed by E^eff = E₀-αV_anod where α is a constant and E₀ = 1.6 eV is a voltage-independent term. The proximity of E₀ to that of oxygen diffusion in titania suggests that oxygen transport across the titania walls at the pore bottoms is the rate-limiting step. These results provide insights into the mechanism of titania nanotube formation and a framework for their rational synthesis for applications.     

Anodic TiO2 nanotubes from stirred baths: hydroxyapatite growth & osteoblast responses

Self-organized porous nano-tubular TiO₂ was anodically formed on titanium from electrolyte containing 1 M Na₂SO₄ and 0.5 wt.% NaF. The oxidation was carried out for 2 h at 20 V and the electrolytes were stirred using (i) magnetic pellet and (ii) ultrasonic waves. The nanotubes have single-pore diameter of 50–90 nm under the magnetic pellet stirring and 75–110 nm under the ultrasonic agitation. The coatings were amorphous in this condition. Amorphous coatings cannot nucleate apatite easily and require crystallization heat-treatments for inducting apatite. The current nanotubes were heated for 2 h at 500 °C. This treatment converted them to crystalline mixed anatase and rutile. These heat-treated coatings were then immersed in simulated body fluid (SBF) for 5, 168 and 504 h to nucleate apatite. Apatite produced on ultrasonically obtained nanotube contained finer crystals.     

Fabrication of high aspect ratio zirconia nanotube arrays by anodization of zirconium foils

The zirconia nanotube arrays with diameter of about 130 nm, a length of up to 190 μm and aspect ratios of more than 1400 were prepared by anodizing a zirconium foil in mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH₄F and 3 wt.% H₂O. The as-prepared nanotube arrays were amorphous zirconia. Monoclinic and tetragonal zirconia coexisted when annealed at 400 °C and 600 °C, while monoclinic zirconia was obtained at 800 °C. The ZrO₂ nanotubes retained their shape after heating up to 800 °C. The lower dissolving rate of zirconia in organic electrolytes might be the main reason for fabrication of zirconia nanotube arrays with high aspect ratio.     

The application of freestanding titanate nanofiber paper for scattering layers in dye-sensitized solar cells

A titanate nanofiber paper with robust and good flexible property was successfully prepared by alkali hydrothermal synthesis with simple paper-making method. These nanofibers were about 80 nm in diameter and had a typical length in the range of tens of micrometers. Despite the transformation from titanate to TiO₂-B phase was initially started, such nanofiber paper still kept its original shape and good flexibility after calcinations at 450 °C for 30 min. A solar cell with titanate nanofiber paper as scattering layer yielded an overall conversion efficiency of 4.90% under an incident solar energy of 100 mW/cm², about 27.5% higher than that without nanofiber paper.     

End-closed NiCoFe-B nanotube arrays by electroless method

A novel approach is obtained during the fabrication of NiCoFe-B nanotube arrays via electroless method. Porous anodic aluminum oxide (AAO) templates fabricated by anodization of aluminum foil were sensitized using PdCl₂ solution and immersed into electroless plating baths at room temperature to produce nanotube arrays. Compositional and morphological properties of the nanotube arrays are characterized. Results indicates the formation of end-closed nanotubes with the dimension of 100-130 nm in outside diameter, which is determined by the pore size of the AAO template, and about 15 nm in thickness of tube walls. The possible formation mechanism of end-closed metallic nanotube arrays is discussed.     

Synthesis and electrophoretic deposition of hydrothermally synthesized multilayer TiO2 nanotubes on conductive filters

TiO₂ nanotubes were synthesized by the decomposition of titanium isopropoxide in water and the calcination at 450 °C for 2 h to form TiO₂ nanoparticles. The synthesized TiO₂ in anatase form nanoparticles were processed hydrothermally in 10 M NaOH solution at 130 °C for 24 h to obtain multilayer TiO₂ nanotubes. TEM analysis revealed that the diameters of the tubes were around 10 nm and they are in the length of 100 nm. Subsequently, colloidal suspensions containing 1% wt. Of TiO2 nanotubes were prepared with TEA and butanol and electrophoretic deposition (EPD) experiments were conducted in order to obtain coatings on Ni and carbon filters using a deposition time of 10 min. and an applied voltage of 65 V. It is also shown that multilayer TiO₂ nanotubes having outer diameter around 10 nm and inner diameters of 4.3 nm can be produced using the described technique. EPD is also shown to be an effective technique to coat three dimensional components, such as Ni and C filters for various applications including water and air purification systems.     

Hydrothermal synthesis of 1D TiO2 nanostructures for dye sensitized solar cells

Mono-dimensional titanium oxide nanostructures (multi-walled nanotubes and nanorods) were synthesized by the hydrothermal method and applied to the construction of dye sensitized solar cells (DSCs). First, nanotubes (TiNTs) and nanotubes loaded with titanium oxide nanoparticles (TiNT/NPs) were synthesized with specific surface areas of 253 m²/g and 304 m²/g, respectively. After that, thermal treatment of the nanotubes at 500 °C resulted in their transformation into the corresponding anatase nanorods (TiNT-Δ and TiNT/NPs-Δ samples). X-ray diffraction and Raman spectroscopy data indicated that titanium oxide in the pristine TiNT and TiNT/NP samples was converted into anatase phase TiO₂ during the heating. Additionally, specific surface areas and water adsorption capacities decreased after the heat treatment due to the sample agglomeration and the collapse of the inner nanotube channels. DSCs were fabricated with the nanotube TiNT and TiNT/NP samples and with the anatase nanorod TiNT-Δ and TiNT/NPs-Δ samples as well. The highest power conversion efficiency of η = 3.12% was obtained for the TiNT sample, despite its lower specific surface compared with the corresponding nanoparticle-loaded sample (TiNT/NP).     

Convenient synthesis of Fe-filled boron nitride nanotubes by SHS method

Fe-filled boron nitride (BN) nanotubes with high purity and good yield were conveniently synthesized by a novel ball-milling and self-propagation high-temperature synthesis (SHS) method at a low temperature (700 °C). The as-prepared product was characterized by XRD, FTIR, SEM, TEM and HRTEM. The results of XRD, FTIR and HRTEM reflect that the product is a hexagonal BN nanotube filled with Fe. The results of SEM and TEM reveal that the Fe-filled BN nanotubes have a diameter of 20-150 nm with the wall-thickness of about 20 nm and the length of more than 5 μm. The possible growth mechanism was also discussed.     

Formation of high bioactive nanoporous titania film by hybrid surface mechanical attrition treatment

Surface mechanical attrition treatment (SMAT) was used to induce grain refinement on the surface of the commercially pure Titanium (Ti). Pure nanocrystalline anatase thin film with nano pores was deposited on pre-SMATed Ti substrate via chemical oxidation by simply soaking in a mix solution of 8.8 mol/l H₂O₂ and 0.1 mol/l HCl at 80 °C for 30 min followed by heat treatment at 400 °C for 1 h in air. The nanoporous anatase showed excellent bioactivity while being soaked in simulated body fluid (SBF), which could be mainly attributed to the unique nanostructure on the SMATed Ti surface. Mechanism of the formation of nanoporous titania was also discussed.     

Fabrication of TiO2 nanotube arrays by electrochemical anodization in an NH4F/H3PO4 electrolyte

Highly ordered TiO₂ nanotube arrays were fabricated by electrochemical anodization of titanium in an NH₄F/H₃PO₄ electrolyte. A TiO₂ crystal phase was identified by X-ray diffraction, and the morphology, length and pore diameter of the TiO₂ nanotube arrays were determined by field-emission scanning electron microscopy (FE-SEM). The anodization parameters including the rate of magnetic stirring, F⁻ concentration, calcination temperature, anodization voltage and anodization time were investigated in detail. The results show that the as-prepared TiO₂ nanotube arrays possessed good uniformity, a well-aligned morphology with a length of 750 nm and an average pore diameter of 62 nm at a 150 rpm rate of magnetic stirring for 120 min at 20 V in an electrolyte mixture of 0.2 M H₃PO₄ and 0.3 M NH₄F with a 500 °C calcination to obtain 100% anatase phase. The adsorption of N-719 dye at different tube lengths was determined by UV-vis analysis and found to increase with increasing tube length. We also discuss the formation mechanism of the TiO₂ nanotube arrays. The findings indicate that the formation of the TiO₂ nanotube arrays proceeds by the combined action of the electrochemical etching and chemical dissolution.