Preparation of near micrometer-sized TiO2 nanotube arrays by high voltage anodization

Highly ordered TiO₂ nanotube arrays with large diameter of 680–750 nm have been prepared by high voltage anodization in an electrolyte containing ethylene glycol at room temperature. To effectively suppress dielectric breakdown due to high voltage, pre-anodized TiO₂ film was formed prior to the main anodizing process. Vertically aligned, large sized TiO₂ nanotubes with double-wall structure have been demonstrated by SEM in detail under various anodizing voltages up to 225 V. The interface between the inner and outer walls in the double-wall configuration is porous. Surface topography of the large diameter TiO₂ nanotube array is substantially improved and effective control of the growth of large diameter TiO₂ nanotube array is achieved. Interestingly, the hemispherical barrier layer located at the bottom of TiO₂ nanotubes formed in this work has crinkles analogous to the morphology of the brain cortex. These structures are potentially useful for orthopedic implants, storage of biological agents for controlled release, and solar cell applications.     

Self-organized highly ordered TiO2 nanotubes in organic aqueous system

A simple method to achieve self-organized, freestanding TiO₂ nanotube array was constructed, free of corrosive etching process which was traditionally employed to separate TiO₂ nanotubes from the metallic Ti substrate. The TiO₂ nanotube arrays were constructed through potentiostatic anodization of Ti foil in aqueous electrolyte containing NH₄F and ethylene glycol. The nanotubes in the array were of 45 μm lengths and 100 nm average pore diameters. The effect of NH₄F concentration on the length of the self-organized nanotube arrays was investigated. Electrochemical and spectroscopic measurements showed that the as-prepared nanotubes possessed large surface areas, good uniformity, and were ready for enzyme immobilization. The as-prepared nanotube arrays were amorphous, but crystallized with annealing at elevated temperatures, as demonstrated by X-ray diffraction (XRD).     

Experimental study on synthesizing TiO2 nanotube/polyaniline (PANI) nanocomposites and their thermoelectric and photosensitive property characterization

In this paper, TiO₂ nanotube/polyaniline (PANI) nanocomposites were made. The thermoelectric and photosensitive properties of the nanocomposites were studied. The effects of processing time, voltage, concentration of F^? ions and H₃PO₄ on the formation of TiO₂ nanotubes were investigated. The morphologies of the synthesized nanocomposites were revealed by scanning electron microscopy (SEM). The formation of polyaniline was confirmed by both Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The optimum conditions for the formation of well-organized TiO₂ nanotubes are at 20 V for 60 min in the electrolyte containing 0.2 M fluorine ions. The highest absolute value of the Seebeck coefficient for the TiO₂ nanotube/polyaniline nanocomposites is 124 μV/K at 30 °C. Pure Ti foil does not show photosensitive property, while the TiO₂ nanotubes have strong photosensitivity.     

Fabrication of partially crystalline TiO2 nanotube arrays using 1, 2-propanediol electrolytes and application in dye-sensitized solar cells

We report the fabrication of self-organized partial crystalline TiO₂ nanotube arrays in 1, 2-propanediol containing fluoride ion. The influence of anodization parameters including NH₄F concentration, water content, anodization voltage and time on the morphology, diameter and length of TiO₂ nanotube were investigated in detail. The prepared TiO₂ nanotube has diameter in 30–120 nm and length in 0.6–3 μm. TiO₂ nanotube arrays are used as photoanode for the application in dye-sensitized solar cell and the photovoltaic performance of 1.91% is achieved with a TiO₂ nanotube sample of 2.2 μm in length combining with N719 dye, and the corresponding photovoltaic parameters of 3.6 mA cm^?2 in short circuit photocurrent density, 840 mV in open circuit potential, and 63.2% in fill factor.     

Facile preparation and photoelectrochemical properties of CdSe/TiO2 NTAs

In the present work we report the design and synthesis of CdSe/TiO₂ nanotube arrays (NTAs) and their implementation as a photoanode for photoelectrochemical (PEC) application. CdSe nanoparticles with well dispersion were decorated on the inner and outer surfaces of 2.5 μm-long TiO₂ nanotubes via electrodeposition. These CdSe/TiO₂ NTAs exhibit a significant photocurrent responds under visible light illumination (λ ≥ 420 nm). The results presented in this study display a promising method that the photoelectrochemical performance could be improved via composition, size and crystalline control of CdSe/TiO₂ NTAs. And the tubular morphology is also able to facilitate charge transport in nanostructure-based PEC cells. This research demonstrates a new approach, which have great potential applications in fabricating novel heterostructure-photoelectrochemical devices.     

The effect of calcination conditions on the morphology,the architecture and the photo-electrical properties of TiO2 nanotube arrays

Highly ordered TiO₂ nanotube arrays were successfully prepared by electrochemical anodization in a formamide-based electrolyte containing 0.5 wt.% NH₄F and 2 vol.% H₂O. The effects of the calcining temperature, the calcining time and the heating rate on the formation of the TiO₂ nanotube arrays were investigated in detail. The morphological changes and phase transformations of the TiO₂ nanotubes were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was found that the calcining temperature and the calcining time determined the crystal phase, while the heating rate was only beneficial to altering the crystallinity. UV–vis diffuse reflectance spectroscopy was used to examine changes in the band gap energy. For applications to dye sensitized solar cells, a maximum conversion efficiency was achieved at 500 °C for 2 h with a heating rate of 10 °C/min, which is attributed to the highly crystalline anatase and the lower surface defect concentrations of the nanotubes. The optimum calcination conditions help to retard the electron recombination and allow higher dye absorption capacities, thereby increasing V_oc and J_sc.     

Electrochemical deposition of iron nanoneedles on titanium oxide nanotubes

Iron as a catalyst has wide applications for hydrogen generation from ammonia, photodecomposition of organics, and carbon nanotube growth. Tuning the size and shape of iron is meaningful for improving the catalysis efficiency. It is the objective of this work to prepare nanostructured iron with high surface area via electrochemical deposition. Iron nanoneedles were successfully electrodeposited on Ti supported TiO₂ nanotube arrays in a chlorine-based electrolyte containing 0.15 M FeCl₂·4H₂O and 2.0 M HCl. Transmission electron microscopic analysis reveals that the average length of the nanoneedles is about 200 nm and the thickness is about 10 nm. It has been found that a high overpotential at the cathode made of Ti/TiO₂ nanotube arrays is necessary for the formation of the nanoneedles. Cyclic voltammetry test indicates that the electrodeposition of iron nanoneedles is a concentration-limited process.     

Characterization of a calcium phosphate–TiO2 nanotube composite layer for biomedical applications

Well-ordered nanotube arrays of titania ~ 0.7 μm high and about 40 or 110 nm in diameter were prepared via electrochemical oxidation at constant voltage (10, 15, 20 or 25 V) in a mixture of 0.86 wt.% of NH₄F, glycerol and deionized water. The effect of annealing the nanotubes at 600 °C on their morphology and structure was examined using SEM and TEM techniques. These substrates are suitable supports for a calcium phosphate coating deposited by a simple immersion in Hank solution.The nucleation and growth of a calcium phosphate (Ca–P) coating deposited on TiO₂ nanotubes (NT) from Hanks' solution was investigated using SEM. XPS and FTIR surface analytical techniques were used to characterize the self-organized porous TiO₂ layers covered with calcium phosphate coatings before and after protein adsorption. Our results confirm that the nanotubular titania layer became stable after annealing at 600 °C, while its internal structure changed from amorphous to crystalline anatase, and eventually, a mixture of anatase and rutile. These thermally stabilized TiO₂ nanotubes significantly enhance apatite formation in Hanks' Balanced Salt Solution as compared to pure Ti covered with a native oxide layer. The Ca–P/TiO₂ NT/Ti surface adsorbs a higher amount of protein (bovine serum albumin, BSA) for a geometric surface area than does the Ti surface. The above difference in protein adsorption suggests a more promising initial cellular response for a Ca–P/TiO₂ NT/Ti composite than for a typical Ti implant surface.     

TiO2 nanotube-based field effect transistors and their application as humidity sensors

TiO₂ nanotubes are the building units of various devices of energy- and environment-related applications and the property studies of individual TiO₂ nanotubes are important to understand and improve the performance of TiO₂ nanotubes-based devices. Here we report the electrical property study of individual TiO₂ nanotubes enabled by the construction of field effect transistors based on individual TiO₂ nanotubes. It is found that individual TiO₂ nanotubes exhibit typical n-type electrical conduction characteristics, with electron mobility of 6.9 × 10^?3 cm²/V s at V_ds = 1 V, and electron concentration of 2.8 × 10¹⁷ cm^?3. Moreover, the on–off ratio of the TiO₂ nanotube-based field effect transistors is as high as 10³. Humidity sensing test shows the sensitive response of the individual TiO₂ nanotubes to water vapor.     

Photocatalytic degradation of gaseous toluene over TiO2–SiO2 composite nanotubes synthesized by sol–gel with template technique

TiO₂–SiO₂ composite nanotubes were successfully synthesized by a facile sol–gel technique utilizing ZnO nanowires as template. The nanotubes were well characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, N₂ adsorption–desorption analysis and UV–vis diffuse reflectance spectroscopy. The nanotubular TiO₂–SiO₂ composite photocatalysts showed diameter of 300–325 nm, fine mesoporous structure and high specific surface area. The results indicated that the degradation efficiency of gaseous toluene could get 65% after 4 h reaction using the TiO₂–SiO₂ composite as the photocatalyst under UV light illumination, which was higher than that of P25.     

Effect of the Ti/Na molar ratio on the acidity and the structure of TiO2 nanostructures: Nanotubes,nanofibers and nanowires

TiO₂ nanotubes were prepared by the hydrothermal treatment of TiO₂ particles with different NaOH concentrations (5, 7, 10 and 12 N) at 140 °C; afterwards, HCl was added until reaching pH 1. Both the crystalline phase and coordination of the TiO₂ nanotubes, composed principally of H₂Ti₃O₇ and H₂Ti₄O₉·2H₂O, were significantly affected by the NaOH rinsing treatment. Likewise, the surface area, pore volume and pore size of the TiO₂ nanotubes changed with the NaOH rinsing treatment. Finally, the NaOH rinsing treatment exerted a notable effect on the generation of Brönsted sites, which is shown by the following sequence: NT7 > NT10 > NT12 > NT5; meanwhile Lewis sites were only present on the NT5 sample.     

Synthesis and crystallization of lead–zirconium–titanate (PZT) nanotubes at the low temperature using carbon nanotubes (CNTs) as sacrificial templates

Pb(Zr_0.52Ti_0.48)O₃ (PZT) nanotubes with diameters of 80–100 nm and a wall thickness of 15–20 nm were prepared by sol–gel template technique and using multi-walled carbon nanotubes (MWCNT) as sacrificial templates. The coating process of MWCNT with PZT precursor sol and removal of the carbon nanotubes by an interrupt heat treatment were discussed and studied by Raman spectroscopy. Simultaneous thermal analysis (STA) revealed that PZT nanotube crystallized at the low temperature of 410 °C by the significantly low activation energy of crystallization of 103.7 kJ/mol. Moreover, based on the X-ray diffraction (XRD) pattern and selected area electron diffraction pattern the crystal structure of the PZT nanotube was determined as perovskite. High resolution transmission electron microscope (HRTEM) and field-emission scanning electron microscope (FE-SEM) images proved that the final PZT had a tubular structure.     

Sonication–hydrothermal combination technique for the synthesis of titanate nanotubes from commercially available precursors

Titanate nanotubes with inner diameters of 2–6 nm, outer diameters of 5–10 nm and lengths up to 600 nm were fabricated by directly using commercial TiO₂ powders as the precursors via sonication–hydrothermal combination approach. The formation processes during sonication treatment under different sonication powers and times and hydrothermal treatment were studied by scanning electron microscope (SEM) and transmission electron microscope (TEM) characterization. The chemical composition of the titanate nanotubes was determined in terms of X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) analysis. The influence of the particle size of the precursors on the formation processes was also examined. The tubular structure of the titanate nanotubes can be remained at the calcination temperature ≤450 °C, but was completely destroyed at high calcination temperature 600 °C.     

Effect of TiO2 nanoparticles aggregation in silicate thin coating films on photocatalytic behavior for antifouling materials

Two types of disperser namely, a high speed agitation bead mill and a colloidal mill, were used for the dispersion behavior control of TiO₂ nanoparticles (20 nm in average primary diameter) in an aqueous suspension. A composite thin film raging in thickness from 90 to 400 nm was prepared from the coating suspension following the addition of a commercial ethyl silicate oligomer binder into the prepared suspension by means of a spray coating method. The mean aggregate size of TiO₂ nanoparticles in the aqueous suspension was found to be 80 nm and 290 nm in diameter, respectively, when using the agitated bead and colloidal milling methods. Large aggregates on the order of several hundred nanometers were found to remain in the suspension after colloidal milling. Further, a fine dispersion of TiO₂ nanoparticles in the thin film produced using the agitation bead milling process promoted the photocatalytic activity and enhanced transparency of the film for visible light. The aggregate structure of TiO₂ nanoparticles in an aqueous suspension was well maintained in the film prepared by a spray coating process.     

Investigation on the effects of PbO content and seeding layers of TiO2 and ZrO2 on the orientation and microstructure of Pb(Zr0.52Ti0.48)O3 ferroelectric films grown by reverse dip-coating method of sol–gel

Lead zirconate titanate Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films were grown on Si (100) and Pt(111)/Ti/SiO₂/Si(100) substrates by a new reverse dip-coating method of sol–gel process. The method was first proposed and applied to coat films. It has several advantages over the conventional sol–gel coating method, including: no consideration of the mechanical transmission that is difficult to manipulate with costly exact apparatus in classical dip-coating procession, convenient processing control, simplicity, low cost, less pollution, and easy fabrication films on large areas and irregular shaped devices etc. This paper studied the factors including PbO content of precursor, TiO₂ and ZrO₂ layers, which are related to raw materials of PZT precursor and influence greatly the crystal orientation of the final thin films. We find that the PZT films deposited by precursor with 20% mole excess Pb displayed strong (111) preferred orientation, with 5% mole excess Pb showed a little (100) orientation and pyrochlore phase. The precursor with 10% mole excess Pb was found prompting the PZT films phase transformation with (110) preferred orientation. In addition, the results show that the TiO₂ and ZrO₂ seeding layers had totally different effects on the preferred orientation of PZT films. The films with TiO₂ seeding layer were highly (111) oriented and exhibited better ferroelectric properties (remnant polarization Pr = 14.2 μC cm^− 2, coercive field Ec = 59.1 Kv cm^− 1) than those of the films with ZrO₂ seeding layer shown (100) orientation (Pr = 7.4 μC cm^− 2, Ec = 42.9 Kv cm^− 1).     

Enhanced bioactivity of self-organized ZrO2 nanotube layer by annealing and UV irradiation

Superhydrophilic ZrO₂ nanotube layer was prepared by anodic oxidation of commercial pure Zr in aqueous solutions containing 1 M (NH₄)₂SO₄ and 0.15 M NH₄F. The effect of annealing and ultraviolet (UV) irradiation treatment on the microstructure, water contact angle and bioactivity of the ZrO₂ nanotube layer was investigated. The as-anodized nanotube layer consists of cubic and amorphous ZrO₂, no apatite crystals are deposited on its surface even after immersion in simulated body fluids (SBF) for 30 days, exhibiting weak apatite-inducing ability. After annealing at 450 °C for 3 h, the nanotube layer is composed of cubic and monoclinic ZrO₂, and its apatite-forming ability is significantly enhanced because of its lattice structure matching that of apatite, apatite can be induced after immersion in SBF for 15 days. UV irradiation of the ZrO₂ nanotube layers does not alter their surface morphologies and phase components, however, can improve the bioactivity only when the ZrO₂ nanotube layer is well crystallized. The enhanced bioactivity by UV irradiation is thought to result from the abundant basic ZrOH groups on the crystallized ZrO₂ nanotube layer. Annealing and UV irradiation treatment do not alter the superhydrophilic nature of the ZrO₂ nanotubes.     

Synthesis and characterization of rice grains like Nitrogen-doped TiO2 nanostructures by electrospinning–photocatalysis

Rice grain-shaped Nitrogen-doped titanium dioxide (N–TiO₂) nano/mesostructures were fabricated through a combination of sol–gel and electrospinning methods. As-spun nanofibers were continuous and upon thermal treatment at 500° C for 1 h in air, the continuous fibers break into rice grain-shaped TiO₂ nanostructures of average diameter 50–80 nm. The nanostructures were characterized by spectroscopy, microscopy and powder X-ray diffraction. The rice grains consist of spherical particles of average diameter of ~ 18 nm and with N doping, their average diameters decrease from ~ 18 to ~ 12 nm. The presence of N in the TiO₂ lattice was confirmed by X-ray photoelectron spectroscopy (XPS). The band-gap of TiO₂ reduced from 3.19 eV to 2.83 eV upon increasing doping level of N from 0% to 5% (w/w), respectively. The N–TiO₂ rice grains showed an enhanced UV light-assisted photocatalysis compared to pure TiO₂ in the photodegradation of Alizarin Red S dye, an industrially important anthraquinone dye.     

The fabrication of SnSe/Ag nanoparticles on TiO2 nanotubes

SnSe and silver (Ag) nanoparticles were sequentially deposited on TiO₂ nanotube (NT) by pulsed electrochemical deposition and polyol chemistry process, respectively. The morphological observation under scanning electron microscope (SEM) showed that the average size of SnSe was about 30 nm and the Ag was about 5 nm. Transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED) examination indicated that Ag nanoparticles exhibited a well-defined crystallinity. However, SnSe nanoparticles were amorphous and they turned to crystalline after being annealed at 300 °C in the atmosphere. The photocatalytic behavior of SnSe/Ag-TiO₂ NT was evaluated by UV–vis diffuse reflectance spectra (DRS). The results showed that the deposition of SnSe and Ag nanoparticles increased light absorption intensity in the wavelength range of visible light, which implied that the SnSe/Ag-TiO₂ NT is a promising ternary hybrid material in photocatalysis.     

Evolution of different structural phases of TiO2 films with oxygen partial pressure and Fe doping and their electrical properties

We have studied the influence of oxygen partial pressure (OPP; 250 mTorr–1 × 10^?5 Torr) and Fe doping (2 and 4 at.%) on structural and electrical properties of TiO₂ thin films on LaAlO₃ substrates. X-ray photoelectron spectroscopy suggests that Fe is not in metal cluster form. It is found that the evolution of the three phases; anatase, rutile and brookite of TiO₂ as well as the magneli phase (Ti_nO_2n?1) strongly depends on the OPP and Fe doping concentration. All the films grown at 250 mTorr show insulating behavior, whereas films grown at 1 × 10^?2 and 1 × 10^?4 Torr reveal high temperature metallic to low temperature semiconducting transition. Interestingly, films deposited at 1 × 10^?5 Torr reveal charge ordering, which is contributed to the magneli phase of TiO₂. The present study suggests that functionality of TiO₂ thin film based devices can be tuned by properly selecting the OPP and dopant concentration.