Investigations on the self-organized growth of TiO2 nanotube arrays by anodic oxidization

The formation mechanism of a thin film of self organized TiO₂ nanotube arrays prepared by anodic oxidization of a pure titanium sheet in electrolyte solutions containing potassium fluoride and sulfate was investigated through near-real time monitoring the anode mass, the current density, and the surface topography during the anodization. Energy dispersive X-ray spectrometry was used to monitor the surface composition change during the anodization. The titanium surface was first electrochemically oxidized to form a layer of dense oxide under which nanotubes were originated. With the protection of the oxide layer, long nanotubes could be formed in electrolyte solutions with relatively high pH. The surface composition analysis indicates that the nanotubes were not totally oxidized to TiO₂. However, no other elements but Ti and O were found in the oxide film. This work reveals a way to fabricate long nanotubes with defined sizes.     

Electron field emission from the carbon-doped TiO2 nanotube arrays

The field-emission characteristics of the carbon-doped TiO₂ nanotube arrays (TNAs), which can be obtained by a heat treatment of the as-fabricated TNAs under a continuous argon and acetylene flux, were investigated. The morphology, crystalline structure, and composition of the as-grown specimens were characterized by the use of field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. It was found that the samples' turn-on electric field is reduced from 21.9 to 5.0 V/μm and the field-emission current density rapidly reaches about 9.0 mA/cm² at 11.8 V/μm after carbon doping. The dramatically improved field-emission characteristics would be mainly attributed to the reduced work function and the enhanced conductivity due to the carbon doping into TNAs.     

Fabrication of 10 nm diameter TiO2 nanotube arrays by titanium anodization

We report the fabrication of TiO₂ nanotube thin films using anodization method with HCl electrolyte and copper cathode. The process represents an alternative electrochemical approach using a non-noble metal cathode along with a safer electrolyte. In addition to the choice of electrolyte, the electrolyte concentration, anodization voltage, and anodization time all affect nanotube morphology. TiO₂ nanotubes with diameters as small as 10 nm were achieved.     

Photocatalytic characteristics of TiO2 nanotubes with different microstructures prepared under different pulse anodizations

In this work, different positive voltages of a pulsed waveform, time intervals and electrolyte stirring were used to prepare by anodization of pure titanium plates, a series of TiO₂ thin films based on nanotubes with different morphologies and dimensions. Electrolyte stirring results in large size TiO₂ nanotubes with uneven surface morphology and less oriented directions. The titanium plates containing the TiO₂ thin films were further annealed at 450 °C for 30 min under air. It was found that only crystalline anatase was formed under this condition. Both methylene blue degradation and antibacterial tests against Escherichia coli were performed to evaluate the photocatalytic performance of these TiO₂ films. Better methylene blue degradation ability was achieved by TiO₂ nanotubes prepared under electrolyte stirring. However, the antibacterial ability of the annealed TiO₂ nanotubes was affected by their inner diameter rather than their length. It is also concluded that the anodized TiO₂ nanotube arrays fabricated in this work are promising for photo-induced methylene blue degradation and bacteria killing applications.     

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.     

Structural and morphological transformations of TiO2 nanotube arrays induced by excimer laser treatment

The structural and morphological transformations of TiO₂ nanotube arrays (TNAs) treated by excimer laser annealing (ELA) were investigated as a function of the laser fluence using parallel and tilted modes. Results showed that the crystallinity of the ELA-treated TNAs reached only about 50% relative to that of TNAs treated by furnace anneal at 400 °C for 1 h. The phase transformation starts from the top surface of the TNAs with surface damage resulting from short penetration depth and limited one-dimensional heat transport from the surface to the bottom under extremely short pulse duration (25 ns) of the excimer laser. When a tilted mode was used, the crystallinity of TNAs treated by ELA at 85° was increased to 90% relative to that by the furnace anneal. This can be attributed to the increased area of the laser energy interaction zone and better heat conduction to both ends of the TNAs.     

Synthesis and Characterization of Niobium-doped TiO2 Nanotube Arrays by Anodization of Ti-20Nb Alloys

Well crystallized niobium-doped TiO₂ nanotube arrays（TiNbO-NT） were successfully synthesized via the anodization of titanium/niobium alloy sheets,followed with a heat treatment at 550 C for 2 h.Morphology analysis results demonstrated that both the titanium/niobium alloy microstructure and the dissolution strength of electrolyte played major roles in the formation of nanotube structure.A single-phase microstructure was more favorable to the formation of uniform nanotube arrays,while modulating the dissolution strength of electrolyte was required to obtain nanotube arrays from the alloys with multi-phase microstructures.X-ray diffraction（XRD） and X-ray photoelectron（XPS） analysis results clearly demonstrated that niobium dopants（Nb 5+） were successfully doped into TiO₂ anatase lattice by substituting Ti 4+ in this approach.     

Dye sensitized solar cells using freestanding TiO2 nanotube arrays on FTO substrate as photoanode

The front-side illuminated dye-sensitized solar cells with TiO₂ nanotube arrays/FTO based photoanode were fabricated by a simple detachment and transfer method. The morphology and crystalline phase of the TiO₂ nanotubes were studied by scanning electron microscopy and X-ray diffraction. A photoconversion efficiency of 1.78% was obtained using TiO₂-nanotube/FTO glass as a photoanode. It was found that the average tubes diameter increased with anodizing voltage, and higher photoconversion efficiency was obtained for nanotubes synthesized at the higher voltage.     

Enhanced photoelectrocatalytic activity in TiO2 nanotube arrays modified with TiO2 nanoparticles

The use of TiO₂ nanotube arrays fabricated by anode oxidation of titanium sheets as a photoelectrocatalyst is limited by low surface activity owing to passive crystallization post-treatment. We report here on a vacuum assisted filling route for modifying TiO₂ nanotube arrays using high-activity anatase TiO₂ nanoparticles as a filling. Photoelectrocatalytic degradation experiments show that a nearly 4-fold activity enhancement in photoelectrocatalysis is achieved and good photoelectrocatalytic stability is kept after the nanotube arrays are filled with the high-activity TiO₂ nanoparticles. The remarkable enhancement in photoelectrocatalysis is ascribed to the key modification of the TiO₂ nanotube arrays using the high-activity TiO₂ nanoparticles. Our findings provide an insight into designing excellent photoelectrocatalysts by filling TNAs with available high-activity TiO₂ nanoparticles.     

Enhanced photoelectrochemical cathodic protection performance of MoS_2/TiO_2 nanocomposites for 304 stainless steel under visible light

In this work,TiO₂nanotube arrays(NTAs)sensitized with MoS₂microspheres(MoS₂/TiO₂nanocomposites)were prepared on a flat Ti substrate via two-step anodization and hydrothermal method sequentially.TiO₂NTAs were composed of many orderly nanotubes,whose large specific surface area was favorable for light absorption and MoS₂microsphere adhesion.The MoS₂microsphere as a narrow band gap semiconductor extended the TiO₂NTAs’absorption band edge to the visible region.The 2D structure of MoS₂microspheres and the construction of heterojunction electronic field at the interface of MoS₂microspheres and TiO₂NTAs promoted the separation of photoinduced carriers.The MoS₂/TiO₂nanocomposites could provide higher photoelectrochemical cathodic protection for 304 stainless steel(304 SS)under visible light than pristine TiO₂NTAs.     

Facile fabrication of three-dimensional interconnected nanoporous N-TiO2 for efficient photoelectrochemical water splitting

Three-dimensional (3D) interconnected porous architectures are expected to perform well in photoelectrochemical (PEC) water splitting due to their high specific surface area as well as favourable porous properties and interconnections. In this work, we demonstrated the facile fabrication of 3D interconnected nanoporous N-doped TiO₂ (N-TiO₂ network) by annealing the anodized 3D interconnected nanoporous TiO₂ (TiO₂ network) in ammonia atmosphere. The obtained N-TiO₂ network exhibited broadened light absorption, and abundant, interconnected pores for improving charge separation, which was supported by the reduced charge transfer resistance. With these merits, a remarkably high photocurrent density at 1.23 V vs. reversible hydrogen electrode (RHE) was realized for the N-TiO₂ network without any co-catalysts or sacrificial reagents, and the photostability can be assured after long term illumination. In view of its simplicity and efficiency, this structure promises for perspective PEC applications.     

ZnO nanowire/TiO2 nanoparticle photoanodes prepared by the ultrasonic irradiation assisted dip-coating method

Hybrid ZnO/TiO₂ photoanodes for dye-sensitized solar cells were prepared by combining ZnO nanowire (NW) arrays and TiO₂ nanoparticles (NPs) with the assistance of the ultrasonic irradiation assisted dip-coating method. Results show that the ultrasonic irradiation was an efficient way to promote the gap filling of TiO₂ NPs in the interstices of ZnO NWs. Hybrid ZnO NW/TiO₂ NP electrodes prepared with ultrasonic treatment exhibited better gap filling efficiency and higher visible absorptance. The overall conversion efficiency of the hybrid electrode was 0.79%, representing 35% improvement compared with that of the traditional one (0.58%). The enlarged surface area and improved attachments of TiO₂ NPs onto the walls of ZnO NWs induced by the application of ultrasonic irradiation may be the underlying reason. Electrochemical impedance spectroscopy measurements indicated that hybrid electrodes combined the advantages of improved electron transport along the ZnO NWs and increased surface area provided by infiltrated TiO₂ NPs, both of which are responsible for the improved cell efficiency.     

Synthesis and characterization of P-doped TiO2 nanotubes

Titanium dioxide (TiO₂) doped with phosphorus (P) was synthesized by anodization of Ti in the mixed acid electrolyte of H₃PO₄ and HF and characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis spectrum. The morphology greatly depends on the applied voltage. The as-formed nanotubes under the optimized condition, at 20 V for 2 h, are highly ordered with ~ 200 nm in length and the average tube diameter is about 100 nm. By annealing the initial samples at different temperatures, the importance of the crystalline nature is confirmed. Significantly, the peak positions of anatase in XRD patterns shifts to lower diffraction angles with an increase in the amount of H₃PO₄ ion. A remarkable red shift of the absorption edge has been observed for the sample formed in the electrolyte of HF and H₃PO₄, which is related to the introduction of P⁵⁺ into TiO₂ crystallization and the possible impurity energy level formed in the TiO₂ band gap. The presence of P 2p state in XPS spectrum can further confirm the P⁵⁺ which can replace a part of Ti⁴⁺ has been introduced into TiO₂ crystallization. The present convenient synthesis technique can be considered to the composition of other doped oxide materials.     

Fabrication of carbon-modified TiO2 nanotube arrays and their photocatalytic activity

TiO₂ nanotube (TN) arrays were fabricated by an anodic oxidation process. Through a heat treatment of the as-fabricated TN arrays under a continuous Ar and acetylene flux, carbon-modified TN (C-TN) arrays were obtained. The as-fabricated catalysts were characterized by FE-SEM, HRTEM, XPS, Raman and UV-Vis spectra. Moreover, photocatalytic activity of the C-TN arrays was evaluated through the photodegradation of aqueous methyl blue. The experiments demonstrated that the C-TN arrays display an excellent photocatalytic activity. Under sunlight irradiation, the C-TN arrays are able to almost completely decompose the methylene blue pollutant of 1 × 10^− 5 M within 300 min.     

The effects of morphological changes on the vibrational properties of self-organized TiO2 nanotubes

This paper reports the synthesis of TiO₂ nanotubes by anodic process. The effects of different electrolyte concentrations on the morphological, structural and vibrational properties of TiO₂ nanotubes were studied. Scanning electron microscopy images showed that nanotube inner diameter ranging from 15 to 115 nm could be obtained. At lower concentration, nanotubes were homogenous, and highly ordered. As concentration increased, nanotubes experienced dissolution process, and led to formation of pore-like structure. X-ray diffraction measurements revealed that anatase phase and Ti were present in the samples upon annealing at 500 °C for one hour. The intensity of Raman E_g(1) peak was electrolyte concentration dependent. Full width half maximum value derived from E_g(1) peak was associated with the intensity which was related to the change of TiO₂ nanotubes morphological quality.     

Anodic TiO2 from stirred Na2SO4/NaF electrolytes: Effect of applied voltage and stirring

TiO₂ was anodically formed on titanium from electrolyte containing 1M Na₂SO₄ and 0.5 wt.% NaF. Oxidation was carried out for 1 h at potentiostatic 20, 30 and 40 V from unstirred as well as stirred baths. At 20 V amorphous and nanotubular oxide is produced irrespective of the stirring conditions. Stirring increased the tubular length compared to the unstirred condition at 20 V. Oxidations at 30 and 40 V produced flat, crystalline anatase under both the bath conditions. A nanotubular network may be formed in the first 5 min and then anodic oxidation may occur laterally, perpendicular to the long-axis of the nanotubes.     

Synthesis of nano B2O3/TiO2 composite material as a new solid phase extractor and its application to preconcentration and separation of cadmium

A new solid phase extractor, nano-scale diboron trioxide/titanium dioxide composite material, was synthesized and used for separation and/or preconcentration of trace cadmium ion from various samples. The characterization of the synthesized material was performed by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffractometer methods (XRD). The specific surface area of the material was also determined and found as 3.4 m²/g. Analytical parameters including pH of sample solution, sample volume, flow rate of sample solution, volume and concentration of eluent for the column solid phase extraction (SPE) procedure were examined. The effect of common matrix ions on the recovery of the cadmium has also been investigated and found that they did not interfere on cadmium preconcentration. Under the optimum experimental conditions, preconcentration factor and analytical detection limit were determined as 50 and 1.44 μg/L, respectively. The reusability (stabile up to 100 run) and adsorption capacity (49 mg/g) of the sorbent were excellent. The accuracy of the method was confirmed by analyzing certified reference materials (Tea leaves GBW-07605). The results demonstrated good agreement with the certified values (relative error <10%). The precision of the method was also satisfactory. The recovery of cadmium under the optimum conditions was found to be 96 ± 3% at 95% confidence level. The method was applied for the determination of cadmium in tap water and tea leaves.     

Hydrophilicity of anatase TiO2/Cr-doped TiO2 thin films with different band gaps

Based on the concept that the electron-hole separation effect caused by a different band-gap structure would improve its hydrophilicity, anatase-TiO₂/Cr-doped TiO₂ thin films were synthesized by DC magnetron sputtering. The optical band gaps of TiO₂ thin films decreased from 3.23 to 2.95 eV with increasing Cr-doping content. Multilayer TiO₂ thin films with different band gaps exhibited a superhydrophilicity under UV illumination. In particular, in anatase TiO₂ (3.23 eV)/4.8% Cr-doped TiO₂ (2.95 eV), the hydrophilicity, which indicated a contact angle of less than 20°, lasted for 48 h in the dark after UV illumination was discontinued. This outstanding result has rarely been reported for TiO₂ thin films, which confirmed that the prominent superhydrophilicity of anatase TiO₂/Cr-doped TiO₂/glass could be attributed to the retardation of electron-hole recombination caused by the band-gap difference.     

Fabrication of TiO2 nanotube film by well-aligned ZnO nanorod array film and sol-gel process

High density TiO₂ nanotube film with hexagonal shape and narrow size distribution was fabricated by templating ZnO nanorod array film and sol-gel process. Well-aligned ZnO nanorod array films obtained by aqueous solution method were used as template to synthesize ZnO/TiO₂ core-shell structure through sol-gel process. Subsequently, TiO₂ nanotube array films survived by removing the ZnO nanorod cores using wet-chemical etching. Polycrystalline anatase TiO₂ nanotube films were ∼ 1.5 μm long and ∼ 100 nm in inter diameter with a wall thickness of ∼ 10 nm.     

Field emission from the structure of well-aligned TiO2/Ti nanotube arrays

Well-aligned TiO₂/Ti nanotube arrays were synthesized by anodic oxidation of titanium foil in 0.5 wt.% HF in various anodization voltages. The images of filed emission scanning electron microscopy indicate that the nanotubes structure parameters, such as diameter, wall thickness and density, can be controlled by adjusting the anodization voltage. The peaks at 25.3° and 48.0° of X-ray diffraction pattern illuminate that the TiO₂ nanotube arrays annealed at 500 °C are mainly in anatase phase. The filed emission (FE) properties of the samples were investigated. A turn-on electric field 7.8 V/µm, a field enhancement factors approximately 870 and a highest FE current density 3.4 mA/cm² were obtained. The emission current (2.3 mA/cm² at 18.8 V/µm) was quite stable within 480 min. The results show that the FE properties of TiO₂/Ti have much relation to the structure parameters.