Growth and characterization of TiO2 nanotubes from sputtered Ti film on Si substrate

In this paper, we present the synthesis of self-organized TiO₂ nanotube arrays formed by anodization of thin Ti film deposited on Si wafers by direct current (D.C.) sputtering. Organic electrolyte was used to demonstrate the growth of stable nanotubes at room temperature with voltages varying from 10 to 60 V (D.C.). The tubes were about 1.4 times longer than the thickness of the sputtered Ti film, showing little undesired dissolution of the metal in the electrolyte during anodization. By varying the thickness of the deposited Ti film, the length of the nanotubes could be controlled precisely irrespective of longer anodization time and/or anodization voltage. Scanning electron microscopy, atomic force microscopy, diffuse-reflectance UV–vis spectroscopy, and X-ray diffraction were used to characterize the thin film nanotubes. The tubes exhibited good adhesion to the wafer and did not peel off after annealing in air at 350 °C to form anatase TiO₂. With TiO₂ nanotubes on planar/stable Si substrates, one can envision their integration with the current micro-fabrication technique large-scale fabrication of TiO₂ nanotube-based devices.     

Synthesis of self-organized mixed oxide nanotubes by sonoelectrochemical anodization of Ti-8Mn alloy

Self ordered arrays of titanium manganese mixed oxide nanotubes were prepared by anodization of Ti8Mn alloy (UNS R56080) under ultrasonication in diluted ethylene glycol containing fluoride. The dimensions of the nanotubes (diameter: 20-100 nm and length: 0.5-2.0 μm) could be tuned by changing the synthesis parameters. The as-anodized nanotubes showed a stoichiometry of (Ti,Mn)O₂. Upon annealing at 500 °C in oxygen atmosphere, the nanotubes contained a mixture of anatase + rutile phases of TiO₂ and Mn₂O₃. The composition of the oxide nanotubes was influenced by the chemistry of the phases present in the alloy. More manganese content was observed in the oxide formed on the β-phase than in the oxide layer of α-phase. Anodization in the ultrasonic field increased the kinetics of nanotubular oxide formation and resulted in homogeneous ordering of the nanotubular arrays as compared to the anodization by conventional stirring in the fluoride containing ethylene glycol solution. Whereas, anodization in aqueous acidified fluoride solutions resulted in severe attack of the β-phase and did not show presence of nanotubular oxide structure.     

Porous niobium oxide films prepared by anodization in HF/H3PO4

Ordered porous niobium oxide with the diameter of less than 10 nm and the aspect ratio of more than 20 is prepared by anodization of niobium foils at 2.5 V in the mixture of 1 wt% HF and 1 M H₃PO₄ for 1 h. In this study, the effects of the mixed electrolytes, anodic potential and anodization time on the preparation of porous niobium oxide are described based on the current-time transients during anodization and morphological observations. It is founded that a single HF electrolyte leads to the formation of pores as well as the fast dissolution of formed pores at the surface. The dissolution of the formed oxide is significantly retarded by the addition of appropriate amount of H₃PO₄.     

An investigation on room temperature synthesis of vertically oriented arrays of iron oxide nanotubes by anodization of iron

Formation of iron oxide nanotubes on to pure iron substrate by an electrochemical anodization method was investigated in fluoride containing electrolytes. Anodization of iron foil in fluoride containing borate solution resulted in stacked nano-ring type oxide morphology. Nanoporous oxide layer was observed at low pH and a granular oxide layer was formed at higher pH of phosphate + fluoride solutions. Formation of either nanoporous or nanotubular oxide layer was observed in ethylene glycol (EG) solution containing 0.05-0.1 M fluoride + 1.5-3.0 vol.% water. Transition from nanoporous structure to nanotubular structure was critically controlled by anodization potential, water addition and fluoride concentration of the EG solution. The potential required for this transition decreased with increase in the water content up to 7 vol.% beyond which enhanced dissolution occurred. Annealing of the nanotubes at 500 °C resulted in predominantly α-Fe₂O₃ crystal structure. The annealed Fe₂O₃ samples consisting of a single layer of nanotubular structure showed a photo current density of 0.4 mA/cm² at 0.5 V Ag/AgCl in 1 M KOH solution under simulated solar light illumination.     

Anodic growth of self-organized anodic TiO2 nanotubes in viscous electrolytes

In this paper, we investigate the electrochemical formation of layers of self-organized high aspect ratio TiO₂ nanotubes, grown by anodization of Ti in viscous electrolytes at different temperatures. In electrolytes consisting of glycerol or ethylene glycol with small additions of fluorides, the nanotube morphology can be strongly influenced by the viscosity, the electrolyte temperature and the applied potential. Compared with water-based electrolytes, in viscous electrolytes much longer and smoother tubes can be grown. Depending on experimental conditions, the nanotubes have an aspect ratio reaching up to about 150 for glycerol and 32 for ethylene glycol. The results further show that in glycerol electrolytes the current efficiency for nanotube formation is close to 100% (this is significantly higher than for comparable water-based electrolytes).     

Electrochemical synthesis of self-organized TiO2 nanotubular structures using an ionic liquid (BMIM-BF4)

We show that an ionic liquid consisting of imidazolium salt with a BF₄ counter ion (BMIM-BF₄) can directly be used to grow well-defined layers of self-organized TiO₂ nanotubes. For this a Ti metal substrate is anodized in this electrolyte for potential range between 3 V_Ag/AgCl and 10 V_Ag/AgCl without addition of free fluoride species (fluorides are used in all previous tube growth procedures). Key factors that influence the morphology and geometry of the resulting nanotubular layer are the anodic potential, the anodization time and particularly the water content in the ionic liquid. The resulting nanotubes layers have thickness in the range of approximately 300-650 nm; with individual tubes that have diameters between 27 nm and 43 nm.     

The effects of growth conditions on the surface properties and photocatalytic activities of anatase TiO<Subscript>2</Subscript> films prepared via electrochemical anodizing and annealing methods

In the present work, nanostructured TiO₂ films were prepared by electrochemical anodization process of titanium in fluoride-containing electrolytes using an innovative approach. After anodization, the TiO₂ films were annealed at 480?°C for 2 h in air in order to acquire anatase phase transformation and increase its crystallinity. The effects of anodization voltage, electrolyte concentration and anodization time on the formation of TiO₂ films and the photocatalytic degradation of methylene blue (MB) were discussed in details. The phase structure and surface morphology of the samples characterized by means of X-ray diffraction and scanning electron microscope. The as-prepared nanostructured TiO₂ film anodized in 0.5% HF electrolyte at 15 V for 240 min showed excellent photocatalytic degradation of MB and is promising for environmental purification.     

Fabrication of high aspect ratio and open‐ended TiO2 nanotube photocatalytic arrays through electrochemical anodization

Well‐aligned, high aspect‐ratio and open‐ended TiO₂ nanotube arrays secured within a Ti foil (TiO₂ nanotubes cartridge) were successfully prepared through the double‐sided anodization method. With ～210 µm of nanotube length, the anodic growth of TiO₂ was accelerated and stabilized by the lactic acid‐containing ethylene glycol electrolyte. In the absence of lactic acid, the anodization led to detachment of nanotubes from the Ti foil after 5–6 h of high voltage (80 V) anodization. Transmission electron microscope image and Raman spectrum revealed that the as‐anodized TiO₂ nanotube arrays without annealing treatment were partially crystalline anatase and demonstrated photocatalytic activity in the mineralization of formic acid. © 2015 American Institute of Chemical Engineers AIChE J, 62: 415–420, 2016     

Enhanced inactivation of E. coli bacteria using immobilized porous TiO2 photoelectrocatalysis

Immobilized TiO₂ nanotube electrodes with high surface areas were grown via electrochemical anodization in aqueous solution containing fluoride ions for photocatalysis applications. The photoelectrochemical properties of the grown immobilized TiO₂ film were studied by potentiodynamic measurements (linear sweep voltammetry), in addition to the calculation of the photocurrent response. The nanotube electrode properties were compared to mesoporous TiO₂ electrodes grown by anodization in sulfuric acid at high potentials (above the microsparking potential) and to 1 g/l P-25 TiO₂ powder. Photocatalyst films were evaluated by high resolution SEM and XRD for surface and crystallographic characterization. Finally, photoelectrocatalytic application of TiO₂ was studied via inactivation of E. coli. The use of the high surface area TiO₂ nanotubes resulted in a high photocurrent and an extremely rapid E. coli inactivation rate of ∼10⁶ CFU/ml bacteria within 10 min. The immobilized nanotube system is proven to be the most potent electrode for water purification.     

Lithium-ion battery anode properties of TiO2 nanotubes prepared by the hydrothermal synthesis of mixed (anatase and rutile) particles

From mixed (anatase and rutile) bulk particles, anatase TiO₂ nanotubes are synthesized in this study by an alkaline hydrothermal reaction and a consequent annealing at 300-400 °C. The physical and electrochemical properties of the TiO₂ nanotube are investigated for use as an anode active material for lithium-ion batteries. Upon the first discharge-charge sweep and simultaneous impedance measurements at local potentials, this study shows that interfacial resistance decreases significantly when passing lithium ions through a solid electrolyte interface layer at the lithium insertion/deinsertion plateaus of 1.75/2.0 V, corresponding to the redox potentials of anatase TiO₂ nanotubes. For an anatase TiO₂ nanotube containing minor TiO₂(B) phase obtained after annealing at 300 °C, the high-rate capability can be strongly enhanced by an isotropic dispersion of TiO₂ nanotubes to yield a discharge capacity higher than 150 mAh g⁻¹, even upon 100 cycles of 10 C-rate discharge-charge operations. This is suitable for use as a high-power anode material for lithium-ion batteries.     

Electrochemically multi-anodized TiO2 nanotube arrays for enhancing hydrogen generation by photoelectrocatalytic water splitting

An enhanced hydrogen production by photoelectrocatalytic water splitting was obtained using extremely highly ordered nanotubular TiO₂ arrays in this work. Highly ordered TiO₂ nanotube arrays with a regular top porous morphology were grown by a facile and green three-step electrochemical anodization. The well ordered hexagonal concaves were uniformly distributed on titanium substrate by the first anodization, served as a template for further growth of TiO₂ nanotubes. As a result, the TiO₂ nanotube arrays constructed through the third anodization showed appreciably more regular architecture than that of the sample by conventional single anodization under the same conditions. The enhanced photoelectrochemical activity was demonstrated through the hydrogen generation by photoelectrocatalytic water splitting, with an exact H₂ evolution rate up to 420 μmol h⁻¹ cm⁻² (10 mL h⁻¹ cm⁻²) in 2 M Na₂CO₃ + 0.5 M ethylene glycol. The photocurrent density of the third-step anodic TiO₂ nanotubes is about 24 mA cm⁻² in 0.5 M KOH, which is 2.2 times higher than that of the normal TiO₂ nanotubes (∼11 mA cm⁻²) by a single electrochemical anodization.     

Electrochemical synthesis of silica-doped high aspect-ratio titania nanotubes as nanobioceramics for implant applications

The preparation of silica-doped high aspect-ratio TiO₂ nanotubes and their apatite-forming ability were demonstrated in this study. The high aspect-ratio TiO₂ nanotube layers were produced by electrochemical anodic oxidation of Ti in chloride-containing electrolytes. Nanotubes were doped with different concentrations of silica particles through anodization in NaCl electrolyte containing different concentrations of water glass (24 g/L or 48 g/L Na₂SiO₃). The biomimetic apatite deposition behavior was evaluated under simulated body fluid (SBF) with an ion concentration nearly equal to human blood plasma. The experimental results collectively demonstrate the successful silica doping of the resultant nanotube layers with significant abundant OH groups on their surfaces. The results of hydroxyapatite (HA) growth on nanotubes clearly show that the silica doping greatly enhances the fast nucleation and growth of HA, especially for the tubes in their “as-formed” amorphous state, which usually require a long time for apatite induction. The nanotubes doped with high silica content combined with an anatase or a mixture of anatase and rutile led to the formation of very thick and continuous apatite layers with a thickness of ∼7 μm in 21 days. In contrast, to the tubes doped with a low concentration of silica (grown in an electrolyte containing 24 g/L Na₂SiO₃), the HA deposited in the form of closely packed spheroid particles and never developed into continuous films. This effect could be attributed to the critical active-site density (silanol groups, >Si-OH), which provides the sterochemical match for apatite growth. Finally, the results of this study provide, for the first time, evidence for the dependence of HA morphology/microstructure on the crystallographic structure and the density of active sites (>Si-OH groups).     

Hierarchical structured TiO2 nano-tubes for formaldehyde sensing

Hierarchical structured TiO₂ nano-tubes were prepared following a two-step method: the highly ordered uniform TiO₂ nanotube arrays were first grown by the conventional electrochemical anodization of the Ti metal sheet followed by mechanical milling of the as-fabricated TiO₂ nanotube arrays. The obtained nanotubes with a length around 400 nm and opening diameter ∼100 nm were formed mixed with the spherical TiO₂ single crystals with a diameter around 10 nm indicating hierarchical nanostructure. The as-synthesized TiO₂ hierarchical nanotubes based resistive-type chemical sensor exhibits good sensitivity to formaldehyde at room temperatures with or without UV-irradiation. The response of the sensor increased almost linearly as a function of the concentration of formaldehyde from 10–50 ppm under UV irradiation. The response of the sensor to different relative humidity and other possible interferents such as ammonia, methanol and alcohol was investigated. The larger response of the sensor to formaldehyde relative to these interferents is suggested to be due to the deeper diffusion of formaldehyde into the TiO₂ nanotubes.     

Synthesis and electrical properties of the (PVA)0.7(KI)0.3·xH2SO4 (0 ≤ x ≤ 5) polymer electrolytes and their performance in a primary Zn/MnO2 battery

Solid acid polymer electrolytes (SAPE) were synthesised using polyvinyl alcohol, potassium iodide and sulphuric acid in different molar ratios by solution cast technique. The temperature dependent nature of electrical conductivity and the impedance of the polymer electrolytes were determined along with the associated activation energy. The electrical conductivity at room temperature was found to be strongly depended on the amorphous nature of the polymers and H₂SO₄ concentration. The ac (100 Hz to 10 MHz) and dc conductivities of the polymer electrolytes with different H₂SO₄ concentrations were analyzed. A maximum dc conductivity of 1.05 × 10⁻³ S cm⁻¹ has been achieved at ambient temperature for electrolytes containing 5 M H₂SO₄. The frequency and temperature dependent dielectric and electrical modulus properties of the SAPE were studied. The charge transport in the present polymer electrolyte was obtained using Wagner's polarization technique, which demonstrated the charge transport to be mainly due to ions. Using these solid acid polymer electrolytes novel Zn/SAPE/MnO₂ solid state batteries were fabricated and their discharge capacity was calculated. An open circuit voltage of 1.758 V was obtained for 5 M H₂SO₄ based Zn/SAPE/MnO₂ battery.     

Control of morphology and composition of self-organized zirconium titanate nanotubes formed in (NH4)2SO4/NH4F electrolytes

We investigated the formation of self-organized zirconium titanate nanotubes by anodizing a Ti-35Zr alloy in 1 M (NH₄)₂SO₄ + 0.1-2.0 wt.% NH₄F electrolytes. The morphology and composition of the zirconium titanate nanotube are controlled by the applied electrochemical conditions. The outer diameter of nanotubes is controlled by the anodization potential in the range between 1 and 100 V (versus Ag/AgCl). Tubes with diameters from 14 to 470 nm can be grown. The nanotube length correlates with the anodic charge up to a length where significant dissolution of the nanotube layer is observed. The wall thickness, composition of the nanotubes and porosity of the nanotube layer are significantly affected by the fluoride ion concentration. The length limiting factor of the nanotube growth is found to be the diffusion of ionic species in the electrolyte.     

Synthesis and growth mechanism of multilayer TiO2 nanotube arrays

High-aspect-ratio TiO(2) nanotube arrays formed by anodic oxidation have drawn extensive attention due to their easy fabrication and various excellent optical, electrical and biomedical properties. In contrast to conventional single-layer TiO(2) nanotubes prepared via constant-voltage anodization, we synthesize multilayer TiO(2) nanotube arrays with high surface area by using alternating-voltage anodization steps. This work presents synthesis and growth mechanisms of single-layer smooth TiO(2) nanotubes, bamboo-type nanotubes and double-layer nanotubes, by tuning various parameters such as voltage, time, and water content in the electrolyte. It is found that ion diffusion inside the nanotubes dominates growth of these three structures. A stable pH and ion-diffusion profile allows the steady growth of smooth TiO(2) tubes in NH(4)F-containing ethylene glycol (EG). The addition of a low-voltage anodization step reduces the pH and ion-diffusion gradient in the nanotubes and induces formation of bamboo-type nanotubes and double-layer nanotubes when a second high-voltage anodization is conducted. Ion diffusion through a nanotube takes time; thus formation of lower-layer TO(2) nanotubes costs more time if longer nanotubes are grown in the upper layer, since ions diffuse through these longer nanotubes. This ion-diffusion controlled growth mechanism is further confirmed by tailoring the water content (0-20 vol%) in the electrolyte and the voltage gaps to control the time needed for initiation of lower-layer TiO(2) nanotube arrays. The fundamental understanding of the growth characteristics of double-layer TiO(2) nanotubes presented in this paper offers us more flexibility in engineering morphology, tuning dimensions and phase compositions of multilayer TiO(2) nanotubes. In addition, we synthesize double-layer TiO(2) nanotube arrays composed of one layer of anatase phase and another layer of amorphous phase.     

Photoelectrochemical oxidation of salicylic acid and salicylaldehyde on titanium dioxide nanotube arrays

We report on the kinetics of photoelectrochemical oxidation of salicylic acid (SA) and salicylaldehyde (SH) on titanium dioxide nanotube arrays. The TiO₂ nanotubes were prepared by the electrochemical oxidation of titanium substrates in a nonaqueous electrolyte (DMSO/HF). Scanning electron microscopy (SEM) was employed to examine the morphology of the formed nanotubes. Linear voltammetry was used to study the electrochemical and photoelectrochemical behavior of the synthesized TiO₂ nanotube arrays. The photoelectrochemical oxidation of SA and SH on the TiO₂ nanotubes was monitored by in situ UV-vis spectroscopy, showing that the kinetics of the photoelectrochemical oxidation of SA and SH follows pseudo first-order and that the rate constant of SH oxidation is 1.5 times larger than that of SA degradation. Quantum chemical calculations based on the DFT method were performed on SA and SH to address the large difference in kinetics. The relatively higher E_LUMO − E_HOMO makes SA more stable and thus more difficult to be oxidized photoelectrochemically. The impact of temperature and initial concentrations on the kinetics of SA and SH photoelectrochemical degradation was also investigated in the present work.     

Photoelectrochemical application of nanotubular titania photoanode

Titania/titanium (TiO₂/Ti) electrodes with tailored surface structure have been fabricated by galvanostatic-potentiostatic anodization process. Highly ordered titania nanotubes array can be prepared by electrolyzing titanium foil at 20 V for 40 min in HF-H₃PO₄ electrolyte. Comparatively, micro-structured and crystallized TiO₂ multiporous film can be prepared at 20-40 V for 6 h in H₂SO₄-H₃PO₄-H₂O₂-HF electrolyte. The morphological characteristics and crystal behaviors of both nanotubular and micro-structured TiO₂ films are investigated by field emission scanning electron microscopy and X-ray diffraction measurement. Photoelectrochemical properties of TiO₂/Ti film electrodes are examined by anodic photocurrent response and cyclic voltammetry measurement. Photocatalytic and photoelectrocatalytic application are investigated by using either nanotubular TiO₂/Ti thin-film or micro-structured TiO₂/Ti thick-film electrodes as photoanodes for recalcitrant organic pollutant degradation.     

A series of LiI/acetamide phase transition electrolytes and their applications in dye-sensitized solar cells

A new series of electrolytes composed of LiI and acetamide have been investigated in dye-sensitized solar cells (DSSCs). These electrolytes melt at about 50 °C and their ionic conductivities vary drastically below and above the melting points (T_m). They tend to form large crystals at low temperature, leading to poor penetration and contact within porous TiO₂ anode film. This shortage is improved by introducing nano-SiO₂ particles into the electrolyte. A total conversion efficiencies (η) of 0.3% at 35 °C and 4.2% at 75 °C are achieved respectively under AM 1.5 simulated solar light illumination when a LiI/acetamide (1:16) electrolyte with 8 wt% nano-SiO₂ is used. It is expected that the DSSC using phase transition electrolyte could show high efficiency for operation at high temperature and high stability for storage at low temperature.     

Formation and characterization of iron oxide nanoparticles loaded on self-organized TiO2 nanotubes

The iron oxide nanoparticles were loaded onto self-organized TiO₂ nanotube layers grown by anodization of Ti in fluoride containing electrolytes. The nanoparticles were obtained by electrodepositing method in glycerol/water/FeCl₃·6H₂O electrolytes at room temperature. The X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) measurements showed that the nanoparticles consisted of iron nanocrystalline (Fe) and magnetite (Fe₃O₄). The hematite (α-Fe₂O₃) structure was obtained by annealing in air at 450 °C. The growth mechanism of the nanoparticles and their morphology were also described. Furthermore, the nanoparticles exhibited good ferromagnetic properties at room temperature.