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.     

Diameter-sensitive biocompatibility of anodic TiO2 nanotubes treated with supercritical CO2 fluid

This work reports on the diameter-sensitive biocompatibility of anodic TiO₂ nanotubes with different nanotube diameters grown by a self-ordering process and subsequently treated with supercritical CO₂ (ScCO₂) fluid. We find that highly hydrophilic as-grown TiO₂ nanotubes become hydrophobic after the ScCO₂ treatment but can effectively recover their surface wettability under UV light irradiation as a result of photo-oxidation of C-H functional groups formed on the nanotube surface. It is demonstrated that human fibroblast cells show more obvious diameter-specific behavior on the ScCO₂-treated TiO₂ nanotubes than on the as-grown ones in the range of diameters of 15 to 100 nm. This result can be attributed to the removal of disordered Ti(OH)₄ precipitates from the nanotube surface by the ScCO₂ fluid, thus resulting in purer nanotube topography and stronger diameter dependence of cell activity. Furthermore, for the smallest diameter of 15 nm, ScCO₂-treated TiO₂ nanotubes reveal higher biocompatibility than the as-grown sample.     

TiO2 micro-flowers composed of nanotubes and their application to dye-sensitized solar cells

TiO₂ micro-flowers were made to bloom on Ti foil by the anodic oxidation of Ti-protruding dots with a cylindrical shape. Arrays of the Ti-protruding dots were prepared by photolithography, which consisted of coating the photoresists, attaching a patterned mask, illuminating with UV light, etching the Ti surface by reactive ion etching (RIE), and stripping the photoresist on the Ti foil. The procedure for the blooming of the TiO₂ micro-flowers was analyzed by field emission scanning electron microscopy (FESEM) as the anodizing time was increased. Photoelectrodes of dye-sensitized solar cells (DSCs) were fabricated using TiO₂ micro-flowers. Bare TiO₂ nanotube arrays were used for reference samples. The short-circuit current (J_sc) and the power conversion efficiency of the DSCs based on the TiO₂ micro-flowers were 4.340 mA/cm² and 1.517%, respectively. These values of DSCs based on TiO₂ micro-flowers were higher than those of bare samples. The TiO₂ micro-flowers had a larger surface area for dye adsorption compared to bare TiO₂ nanotube arrays, resulting in improved J_sc characteristics. The structure of the TiO₂ micro-flowers allowed it to adsorb dyes very effectively, also demonstrating the potential to achieve higher power conversion efficiency levels for DSCs compared to a bare TiO₂ nanotube array structure and the conventional TiO₂ nanoparticle structure.     

Effect of TiO2 nanotubes with TiCl4 treatment on the photoelectrode of dye-sensitized solar cells

In this study, we used the electrochemical anodization to prepare TiO₂ nanotube arrays and applied them on the photoelectrode of dye-sensitized solar cells. In the field emission scanning electron microscopy analysis, the lengths of TiO₂ nanotube arrays prepared by electrochemical anodization can be obtained with approximately 10 to 30 μm. After titanium tetrachloride (TiCl₄) treatment, the walls of TiO₂ nanotubes were coated with TiO₂ nanoparticles. XRD patterns showed that the oxygen-annealed TiO₂ nanotubes have a better anatase phase. The conversion efficiency with different lengths of TiO₂ nanotube photoelectrodes is 3.21%, 4.35%, and 4.34% with 10, 20, and 30 μm, respectively. After TiCl₄ treatment, the efficiency of TiO₂ nanotube photoelectrode for dye-sensitized solar cell can be improved up to 6.58%. In the analysis of electrochemical impedance spectroscopy, the value of R_k (charge transfer resistance related to recombination of electrons) decreases from 26.1 to 17.4 Ω when TiO₂ nanotubes were treated with TiCl₄. These results indicate that TiO₂ nanotubes treated with TiCl₄ can increase the surface area of TiO₂ nanotubes, resulting in the increase of dye adsorption and have great help for the increase of the conversion efficiency of DSSCs.     

Review of titania nanotubes: Fabrication and cellular response

Titania (TiO₂) nanotube is gaining prominence as an implantation material due to its unique properties such as high specific surface area and the ability to exhibit positive cellular response. In this paper, we briefly review the current state of fabrication methods to synthesize nanotubular TiO₂ surface topography, and discuss its effect on cellular response of different cells in terms of cell adhesion, proliferation and differentiation. In vitro and in vivo studies by using TiO₂ nanotubes are also presented establishing the potential of nanotubes in biomedical applications. Finally, an outlook of future growth of research in TiO₂ nanostructures beyond the nanotubes is provided     

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.     

Synthesis and electrochemical properties of CeO2 nanoparticle modified TiO2 nanotube arrays

In this paper, a cerium dioxide (CeO₂) modified titanium dioxide (TiO₂) nanotube array film was fabricated by electrodeposition of CeO₂ nanoparticles onto an anodized TiO₂ nanotube array. The structural investigation by X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated that the CeO₂ nanoparticles grew uniformly on the walls of the TiO₂ nanotubes. The composite was composed of cubic-phase CeO₂ crystallites and anatase-phase TiO₂ after annealing at 450 °C. The cyclic voltammetry and chronoamperometric charge/discharge measurement results indicated that the CeO₂ modification obviously increased the charge storage capacity of the TiO₂ nanotubes. The charge transfer process at the surface, that is, the pseudocapacitance, was the dominate mechanism of the charge storage in CeO₂-modified TiO₂ nanotubes. The greater number of surface active sites resulting from uniform application of the CeO₂ nanoparticles to the well-aligned TiO₂ nanotubes contributed to the enhancement of the charge storage density.     

Crystalline phase of TiO2 nanotube arrays on Ti–35Nb–4Zr alloy: Surface roughness,electrochemical behavior and cellular response

An investigation was made into the electrochemical, structural and biological properties of self-organized amorphous and anatase/rutile titanium dioxide (TiO₂) nanotubes deposited on Ti–35Nb–4Zr alloy through anodization-induced surface modification. The surface of as-anodized and heat-treated TiO₂ nanotubes was analyzed by field emission scanning electron microscopy (FE-SEM), revealing morphological parameters such as tube diameter, wall thickness and cross-sectional length. Glancing angle X-ray diffraction (GAXRD) was employed to identify the structural phases of titanium dioxide, while atomic force microscopy (AFM) was used to measure surface roughness associated with cell interaction properties. The electrochemical stability of TiO₂ was examined by electrochemical impedance spectroscopy (EIS) and the results obtained were correlated with the microstructural characterization. The in vitro bioactivity of as-anodized and crystallized TiO₂ nanotubes was also analyzed as a function of the presence of different TiO₂ polymorphic phases. The results indicated that anatase TiO₂ showed higher surface corrosion resistance and greater cell viability than amorphous TiO₂, confirming that TiO₂ nanotube crystallization plays an important role in the material's electrochemical behavior and biocompatibility.     

Morphology and Microstructure of As-Synthesized Anodic TiO2 Nanotube Arrays

The as-grown structure of electrochemically synthesized titania nanotube arrays is investigated by scanning electron microscope (SEM) in combination with transmission electron microscope (TEM) as well as X-ray diffraction (XRD). The analysis reveals a preferred growth direction of the nanotubes relative to the substrate surface and the well control on the nanotube arrays morphology. The crystal structure of the anatase phase is detected and exists in the tube walls without any thermal treatment, which makes it possible to realize the application of as-formed TiO₂ nanotubes avoiding the degradation of the nanotube structures when sintering. In addition, a new growth, layered model of the anodic TiO₂ nanotubes is presented to obtain further understanding of the growth mechanism.     

Efficient synthesis of titania nanotubes and enhanced photoresponse of Pt decorated TiO2 for water splitting

We investigated the effect of HMT (hexamethylenetetraamine) on the anodic growth of TiO₂ nanotube arrays. The tube length increases to 4.3 μm with HMT concentration increasing to 0.04 mol·L⁻¹. Adsorption of HMT on the TiO₂ surface is shown to markedly decrease the chemical dissolution rate of tube mouth, resulting in longer nanotube length. Furthermore, Pt nanoparticles were successfully deposited on the surface of TiO₂ nanotubes by ac electrodeposition method. The TiO₂/Pt composites were characterized by field emission scanning electron microscope (FESEM), X-ray photoelectron spectra (XPS), and photoelectrochemistry. An enhancement in photocurrent density has been achieved upon modification of TiO₂ nanotubes with Pt nanoparticles.     

Silicon-Doped Titanium Dioxide Nanotubes Promoted Bone Formation on Titanium Implants

While titanium (Ti) implants have been extensively used in orthopaedic and dental applications, the intrinsic bioinertness of untreated Ti surface usually results in insufficient osseointegration irrespective of the excellent biocompatibility and mechanical properties of it. In this study, we prepared surface modified Ti substrates in which silicon (Si) was doped into the titanium dioxide (TiO₂) nanotubes on Ti surface using plasma immersion ion implantation (PIII) technology. Compared to TiO₂ nanotubes and Ti alone, Si-doped TiO₂ nanotubes significantly enhanced the expression of genes related to osteogenic differentiation, including Col-I, ALP, Runx2, OCN, and OPN, in mouse pre-osteoblastic MC3T3-E1 cells and deposition of mineral matrix. In vivo, the pull-out mechanical tests after two weeks of implantation in rat femur showed that Si-doped TiO₂ nanotubes improved implant fixation strength by 18% and 54% compared to TiO₂-NT and Ti implants, respectively. Together, findings from this study indicate that Si-doped TiO₂ nanotubes promoted the osteogenic differentiation of osteoblastic cells and improved bone-Ti integration. Therefore, they may have considerable potential for the bioactive surface modification of Ti implants.     

Fabrication of TiO2 nanotubes by using electrodeposited ZnO nanorod template and their application to hybrid solar cells

Vertically aligned TiO₂ nanotubes have been fabricated on the indium-doped tin oxide (ITO) by a simple and versatile technique using the electrochemically deposited ZnO nanorods, oriented along the c-axis, as a template in the spin-on based sol-gel reaction of a Ti precursor. The diameter, length, and shape of TiO₂ nanotubes were controlled by changing the initial ZnO nanorod template and the spin conditions during sol-gel process of a Ti precursor. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were used to confirm the successful formation of TiO₂ nanotubes and characterize their structure and morphology. Furthermore, as an application of the TiO₂ nanotubes, hybrid solar cells based on TiO₂ and poly[2-methoxy,5-(2′-ethyl-hexyloxy)1,4-phenylenevinylene] (MEH-PPV) were successfully fabricated.     

An innovative approach to electro-oxidation of dopamine on titanium dioxide nanotubes electrode modified by gold particles

Au/TiO₂/Ti electrodes were prepared by galvanic deposition of gold particles from an acidic bath containing KAu(CN)₂ in the presence of a citrate buffer onto TiO₂ nanotubes layer on titanium substrates. Titanium oxide nanotubes were fabricated by anodizing titanium foil in a DMSO fluoride-containing electrolyte. The morphology and surface characteristics of Au/TiO₂/Ti electrodes were investigated using scanning electron microscopy and energy-dispersive X-ray, respectively. The results indicated that gold particles were homogeneously deposited on the surface of TiO₂ nanotubes. The nanotubular TiO₂ layers consist of individual tubes of about 40–80 nm diameters. The electro-catalytic behavior of Au/TiO₂/Ti electrodes for the dopamine electro-oxidation was studied by cyclic voltammetry and differential pulse voltammetry. The results showed that Au/TiO₂/Ti electrodes exhibit a considerably higher electro-catalytic activity toward the oxidation of dopamine. The catalytic oxidation peak current showed a linear dependence on dopamine concentration and a linear calibration curve was obtained in the concentration range of 0.5–2.5 mM of dopamine.     

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.     

N-doped anodic titania nanotube arrays for hydrogen production

Titanium dioxide (TiO₂) nanotube arrays are grown in a mixed electrolyte by anodizing process. The anodic nanotubes for N-doping were calcinated at 773 K in a tube furnace with a mixture of NH₃ and Ar gas. The photocatalytic activity of N-doped TiO₂ nanotubes was carried out in a water-splitting reaction under UV and visible light irradiation. Various characterization techniques (Scanning electron microscopy, X-ray diffractometry, X-ray photo-electron spectroscopy, etc.) are used to study the surface morphology, phase of structure, and binding energy.     

Modified microwave method for the synthesis of visible light-responsive TiO2/MWCNTs nanocatalysts

Recently, TiO₂/multi-walled carbon nanotube (MWCNT) hybrid nanocatalysts have been a subject of high interest due to their excellent structures, large surface areas and peculiar optical properties, which enhance their photocatalytic performance. In this work, a modified microwave technique was used to rapidly synthesise a TiO₂/MWCNT nanocatalyst with a large surface area. X-ray powder diffraction, field-emission scanning electron microscopy, transmission electron microscopy and Brunauer-Emmett-Teller measurements were used to characterise the structure, morphology and the surface area of the sample. The photocatalytic activity of the hybrid nanocatalysts was evaluated through a comparison of the degradation of methylene blue dye under irradiation with ultraviolet and visible light. The results showed that the TiO₂/MWCNT hybrid nanocatalysts degraded 34.9% of the methylene blue (MB) under irradiation with ultraviolet light, whereas 96.3% of the MB was degraded under irradiation with visible light.     

Ni-doped TiO2 nanotubes for wide-range hydrogen sensing

Doping of titania nanotubes is one of the efficient way to obtain improved physical and chemical properties. Through electrochemical anodization and annealing treatment, Ni-doped TiO₂ nanotube arrays were fabricated and their hydrogen sensing performance was investigated. The nanotube sensor demonstrated a good sensitivity for wide-range detection of both dilute and high-concentration hydrogen atmospheres ranging from 50 ppm to 2% H₂. A temperature-dependent sensing from 25°C to 200°C was also found. Based on the experimental measurements and first-principles calculations, the electronic structure and hydrogen sensing properties of the Ni-doped TiO₂ with an anatase structure were also investigated. It reveals that Ni substitution of the Ti sites could induce significant inversion of the conductivity type and effective reduction of the bandgap of anatase oxide. The calculations also reveal that the resistance change for Ni-doped anatase TiO₂ with/without hydrogen absorption was closely related to the bandgap especially the Ni-induced impurity level.     

Photoelectrocatalytic properties of Ag nanoparticles loaded TiO2 nanotube arrays prepared by pulse current deposition

A pulse current deposition technique was adopted to construct highly dispersed Ag nanoparticles on TiO₂ nanotube arrays which were prepared by the electrochemical anodization. The morphology, crystallinity, elemental composition, and UV-vis absorption of Ag/TiO₂ nanotube arrays were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and diffuse reflectance spectra (DRS). In particular, the photoelectrochemical properties and photoelectrocatalytic activity under UV light irradiation and the photocatalytic activity under visible light irradiation for newly synthesized Ag/TiO₂ nanotube arrays were investigated. The maximum incident photon to charge carrier efficiency (IPCE) value of Ag/TiO₂ nanotube arrays was 51%, much higher than that of pure TiO₂ nanotube arrays. Ag/TiO₂ nanotube arrays exhibited higher photocatalytic activities than the pure TiO₂ nanotube arrays under both UV and visible light irradiation. The photoelectrocatalytic activity of Ag/TiO₂ nanotube arrays under UV light irradiation was 1.6-fold enhancement compared with pure TiO₂ nanotube arrays. This approach can be used in synthesizing various metal-loaded nanotube arrays materials.     

Electrochemical characterization of TiO2/WOx nanotubes for photocatalytic application

TiO₂/WO_x nanotubes have unique photo-energy retention properties that have gathered scientific interest. Herein, we report the synthesis, morphological characterization, and the electrochemical characterization of TiO₂/WO_x nanotubes compared with pure TiO₂ nanotubes, prepared by anodization technique. Significant structural differences were not observed in TiO₂/WO_x nanotubes as observed by using scanning electron microscopy and transmission electron microscopy. The charge transfer resistance of TiO₂/WO_x before and after photo irradiation determined by using electrochemical impedance spectroscopy proves the inherent energy retention property which was not observed in pure TiO₂ nanotubes.     

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.