Oriented TiO2 Nanotube Research Articles

Electrochemical Method for Synthesis of a ZnFe2O4/TiO2 Composite Nanotube Array Modified Electrode with Enhanced Photoelectrochemical Activity

AbstractAn electrode with intimate and well‐aligned ZnFe2O4/TiO2 composite nanotube arrays is prepared via electrochemical anodization of pure titanium foil in fluorine‐containing ethylene glycol, followed by a novel cathodic electrodeposition method. The deposition of ZnFe2O4 is promoted in the self‐aligned, vertically oriented TiO2 nanotube arrays but minimized at the tube entrances. Thus, pore clogging is prevented. Environmental scanning electron microscopy, energy‐dispersive X‐ray spectra, high‐resolution transmission electron microscopy, X‐ray diffraction patterns, and X‐ray photoelectron spectroscopy indicate that the as‐prepared samples are highly ordered and vertically aligned TiO2 nanotube arrays with ZnFe2O4 nanoparticles loading. The TiO2 nanotubes are anatase with the preferential orientation of <101> plane. Enhanced absorption in both UV and visible light regions is observed for the composite nanotube arrays. The current–voltage curve of ZnFe2O4‐loaded TiO2 nanotube arrays reveals a rectifying behavior. The enhanced separation of photoinduced electrons and holes is demonstrated by surface photovoltage and photocurrent measurements. Meanwhile, the photoelectrochemical investigations verify that the ZnFe2O4/TiO2 composite nanotube array modified electrode has a more effective photoconversion capability than the aligned TiO2 nanotube arrays alone. In addition, the photoelectrocatalytic ability of the novel electrode is found enhanced in the degradation of 4‐chlorophenol.

Morphology dependence of TiO2 nanotube arrays on anodization variables and buffer medium

Vertically oriented TiO2 nanotube arrays were prepared by potentiostatic anodization of Ti foils in HF/acetic acid (HAC) aqueous solution. Anodization variables including anodization electrolyte concentration, anodization voltage, anodization time and buffer medium can be chosen and adjusted to manipulate the nanotube arrays to give the required length and morphology.

Preparation and Optical Properties of TiO<sub>2</sub> Nanotube Arrays by Anodic Oxidation Method

Titania nanotube arrays were fabricated by anodic oxidation of a pure titanium foil. The morphology, structure and optical properties of the nanotube arrays were characterized by SEM, XRD and UV-Vis spectrum. The effect of anodic oxidation parameters on the morphology was investigated. The growth mechanism of the titania nanotube arrays was discussed. The results show that the vertical oriented TiO2 nanotube arrays can be obtained at 20V for 60min in 0.5wt%HF+1mol/L H3PO4 solution. The aperture size of nanotube is 60nm~80nm and the thickness of tube wall is about 10nm. The spectral absorptivity of TiO2 nanotube arrays membrane calcined at 600°C towards ultravlolet light 349.7nm and visible light 443.9nm was relatively higher. It is also found that the crystal structure is greatly affected by calcining temperature.

Application of Vertically Oriented TiO<sub>2</sub> Nanotube Arrays on Dye Sensitized Solar Cells

In this research, vertically oriented TiO2 nanotube was fabricated on ITO glass by electrochemistry method, and investigated by the measurements of X-ray diffractometer (XRD), scanning electron microscopy (SEM), and ultraviolet-visible (UV/VIS) spectrometer. X-ray diffraction patterns show that the best sintering temperature of TiO2 nanotube is 500°C, at which TiO2 anatase phase forms best. SEM images reveal that the diameter and height of TiO2 nanotube are 70 nm and 230 nm, respectively. Transmittance curves reveal that the transmittance of annealed TiO2 nanotube is about 80%~90%, and is obviously higher than non-annealed TiO2 nanotube. The absorption band of annealed TiO2 nanotube is at 330~370 nm. The results of current-voltage (I-V) characteristics analysis reveal that dye-sensitized solar cell with TiO2 nanotube electrode has better I-V characteristics and efficiency than TiO2 film electrode. This result may be due to the annealed TiO2 nanotube applied on the electrode of dye-sensitized solar cell can increase the contact area between TiO2 and dye, resulting in the enhancement of I-V characteristics and efficiency for dye-sensitized solar cell.

Synthesis and applications of electrochemically self-assembled titania nanotube arrays

Highly ordered vertically oriented TiO(2) nanotube arrays fabricated by electrochemical anodization offer a large surface area architecture with precisely controllable nanoscale features. These nanotubes have shown remarkable properties in a variety of applications including, for example, their use as hydrogen sensors, in the photoelectrochemical generation of hydrogen, dye-sensitized and solid-state heterojunction solar cells, photocatalytic reduction of carbon dioxide into hydrocarbons, and as a novel drug delivery platform. Herein we consider the development of the various nanotube array synthesis techniques, different applications of the TiO(2) nanotube arrays, unresolved issues, and possible future research directions.

Facile fabrication, characterization, and enhanced photoelectrocatalytic degradation performance of highly oriented TiO2 nanotube arrays

Highly ordered TiO2 nanotube arrays were successfully fabricated using ethanol and water mixture electrolytes (40 vol% ethanol and 0.2 wt% hydrofluoric acid) by a facile electrochemical anodization method. The as-prepared nanotube arrays were grown perpendicular to the titanium substrate with about 90 nm in diameter, 20 nm in wall thickness, and around 500 nm in length. The formation mechanism of the samples is briefly discussed. A blue shift in the spectrum of UV–Vis absorption was observed with respect to a piece of the sol–gel derived TiO2 film. Moreover, photocurrent response and photoelectrocatalytic degradation of methyl orange under ultraviolet light irradiation were adopted to evaluate the photoelectrocatalytic properties of the TiO2 nanotube arrays. We demonstrate that the highly ordered TiO2 nanotube arrays possess much better photoelectrocatalytic activity than the sol–gel derived TiO2 film and good stability.

Vertically Oriented TiO2 Nanotube Arrays Grown on Ti Meshes for Flexible Dye-Sensitized Solar Cells

We describe here the growth of vertically oriented TiO2 nanotube arrays on Ti meshes by electrochemical anodization and their application as flexible electrodes for dye-sensitized solar cells (DSC). The dependence of physical features (i.e., diameter, wall thickness, and length) of TiO2 nanotube arrays on the anodization duration is systematically studied. Our results indicate that this type of flexible electrodes features transparency and high bend ability when subjected to external force. Varying the length of nanotube arrays has been demonstrated to critically influence the solar-to-electric conversion efficiency of DSCs. In combination with a low-volatility organic electrolyte and a rigid platinized conductive glass counter electrode, TiO2 nanotube arrays with an optimized length of 40 μm on a 50-mesh substrate achieve a conversion efficiency of 1.47% (calculated from the geometric area of Ti mesh) at 97.4 mW/cm2 AM 1.5 simulated full light. A lightweight flexible DSC is also fabricated using the same nanotube arrays combined with a flexible Pt/ITO/PET counter electrode, which generates a conversion efficiency of 1.23%.

Synthesis and characterization of anodized titanium-oxide nanotube arrays

Anodized titanium-oxide containing highly ordered, vertically oriented TiO2 nanotube arrays is a nanomaterial architecture that shows promise for diverse applications. In this paper, an anodization synthesis using HF-free aqueous solution is described. The anodized TiO2 film samples (amorphous, anatase, and rutile) on titanium foils were characterized with scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. Additional characterization in terms of photocurrent generated by an anode consisting of a titanium foil coated by TiO2 nanotubes was performed using an electrochemical cell. A platinum cathode was used in the electrochemical cell. Results were analyzed in terms of the efficiency of the current generated, defined as the ratio of the difference between the electrical energy output and the electrical energy input divided by the input radiation energy, with the goal of determining which phase of TiO2 nanotubes leads to more efficient hydrogen production. It was determined that the anatase crystalline structure converts light into current more efficiently and is therefore a better photocatalytic material for hydrogen production via photoelectrochemical splitting of water.

Improved Photocatalytic Degradation of Textile Dye Using Titanium Dioxide Nanotubes Formed Over Titanium Wires

Titanium dioxide (TiO2) nanotubes formed by anodization over titanium wires show a significant improvement in photocatalytic activity compared to the nanotubes formed over foils. This is evident when the fractional conversion of a textile dye, methyl orange, increased from 19% over a foil to 40% over wires in the presence of nanotubes of identical dimensions illuminated over the same geometrical area. Higher degradation rates with Pt-TiO2 nanotubes over foils are matched by the Pt-free TiO2 nanotubes over the wires. The higher photocatalytic activity over the anodized wires can be attributed to the efficient capture of reflected and refracted light by the radially outward oriented TiO2 nanotubes formed over the circumference of the titanium wire. The formation of TiO2 nanotubes over wires can be considered as an effective alternate to improve photodegradation rates by avoiding expensive additives.

Effect of Incorporation of TiO2 Nanoparticles into Oriented TiO2 Nanotube Based Dye-Sensitized Solar Cells

The authors present the first reported use of vertically oriented titanium oxide nanotube/titanium oxide nanoparticle nanocomposites to improve the efficiency of TiO2 based dye-sensitized solar cells (DSSCs). Titanium oxide nanotubes incorporating titanium oxide nanoparticles were prepared to harvest more solar light, and their solar-to-electric conversion efficiency was characterized. The incorporation of the titanium oxide nanoparticles increased the visible light absorption of the titanium oxide nanotube based DSSC system because of the increased light absorption resulting from the increased surface area. However, the photocurrent enhancement was more obvious under the illumination of visible light with a wavelength longer than 550 nm owing to the light scattering effect of the titanium oxide nanoparticles.

Photoeletrocatalytic Activity of a Cu2O-Loaded Self-Organized Highly Oriented TiO2 Nanotube Array Electrode for 4-Chlorophenol Degradation

Differently sized Cu2O nanoparticles have been assembled photocatalytically on the surface of self-organized highly oriented TiO2 nanotubes obtained by anodization of a Ti sheet in fluoride-containing electrolytes. X-ray diffraction analysis identifies an anatase structure and fine preferential orientation of (101) planes. The UV-vis absorption edge of the TiO2 nanotube arrays shift to lower energy after Cu2O loading. The composite array electrode exhibits a higher photovoltage response than the TiO2 powders directly deposited on a Ti sheet. The highest photoconversion efficiencies observed for the Cu2O-loaded electrode are 17.2% and 0.82% under UV light and visible light irradiation, respectively. Especially, the composite array electrode shows a higher efficiency than the nonloaded one for the photoelectrocatalytic decomposition of 4-chlorophenol. The improved photoeletrocatalytic activity of the TiO2/Cu2O composite array electrode is attributed to the synergistic effect of Cu2O nanoparticles and TiO2 nanotube arrays. The Cu2O nanoparticles could enhance the efficiency of photon harvesting and reduce the chances of electron-hole recombination by sending the electrons to the conduction band of TiO2 nanotubes. The accumulated electrons in the conduction band of TiO2 nanotubes would reduce oxygen to form peroxides for enhanced advanced oxidation. The byproducts were identified by high-performance liquid chromatography.

Functionalization of Self-Organized TiO2 Nanotubes with Pd Nanoparticles for Photocatalytic Decomposition of Dyes under Solar Light Illumination

Self-organized, vertically oriented TiO2 nanotube arrays prepared by the sonoelectrochemical anodization method are functionalized with palladium (Pd) nanoparticles of approximately 10 nm size. A simple incipient wetness method is adopted to distribute the Pd nanoparticles uniformly throughout the TiO2 nanotubular surface. This functionalized material is found to be an excellent heterogeneous photocatalyst that can decompose nonbiodegradable azo dyes (e.g., methyl red and methyl orange) rapidly (150-270 min) and efficiently (100%) under ambient conditions using simulated solar light in the absence of any external oxidative radicals such as hydrogen peroxide.

Direct electrochemistry of horseradish peroxidase on TiO2 nanotube arrays via seeded-growth synthesis

Horseradish peroxidase (HRP) was successfully immobilized on vertically oriented TiO2 nanotube arrays (NTAs), which was prepared by a seeded-growth mechanism. The nanotubular structure of TiO2 was characterized by scanning electron microscope (SEM). After encapsulated HRP on TiO2 nanotube arrays, the direct electron transfer of HRP was observed. Owing to the redox reaction of electroactive center of HRP, the HRP/TiO2 NTAs modified electrode exhibited a pair of quasi-reversible peaks with the peak-to-peak separation of 70mV and the formal potential of −0.122V (vs. SCE) in 0.2molL−1 phosphate buffer solution (PBS, pH 7.0). The number of transference electron was 0.84 and the direct electron transfer (ET) constant (ks) was 3.82s−1. The HRP/TiO2 NTAs modified electrode displayed an excellent electrocatalytic performance for H2O2 and the formal Michaelis–Menten constant (Kmapp) was 1.9mmolL−1. The response currents had a good linear relation with the concentration of H2O2 from 5.0×10−7molL−1 to 1.0×10−5molL−1 and 5.0×10−5molL−1 to 1.0×10−3molL−1, respectively.

Vertically Oriented Titania Nanotubes Prepared by Anodic Oxidation on Si Substrates

Vertically oriented titania nanotube arrays were fabricated by anodization of titanium film deposited on silicon substrates under different processing conditions. The anodic formation of nanoporous titania on silicon substrate was investigated in aqueous solutions mixed with highly corrosive Na2SO4/NaF/citric acid. In the result of the anodization of titanium film deposited at room temperature, a very thin layer of ~70 nm having a worm-like structure was grown on the top of the porous layer. But, in the case of titanium film deposited at 500deg, vertically oriented TiO2 nanotube arrays were formed. The average tube outer diameter of the nanotube was 74 nm to 100 nm. The longest nanotube of 681 mum was obtained at 15 V and 30 min. The current density transient curve recorded during anodization under a constant voltage showed a typical behavior for self-organized pore formation.

High Photoconductive Vertically Oriented TiO2 Nanotube Arrays and Their Composites with Copper Phthalocyanine

Size-controllable and vertically oriented TiO2 nanotube arrays were fabricated by using anodic oxidation of pure titanium sheets in electrolyte solutions and were characterized by field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). A double-layered photoconductive device (Au/copper phthalocyanine/TiO2 nanotubes/titanium) was designed and fabricated based on the TiO2 nanotube arrays. The I−V curves demonstrated typical heterojunction characteristics and remarkable photoconductive performance for the devices. The effect of the thickness of copper phthalocyanine (CuPc) and the diameter of TiO2 nanotubes on the photoconductive property of devices was studied. The current value increased by 2 or 3 orders of magnitude when exposed to light from the dark, depending on the thickness of the CuPc layer and the diameter of TiO2 nanotubes.

Removing Structural Disorder from Oriented TiO2 Nanotube Arrays: Reducing the Dimensionality of Transport and Recombination in Dye-Sensitized Solar Cells

We report on the influence of morphological disorder, arising from bundling of nanotubes (NTs) and microcracks in films of oriented TiO2 NT arrays, on charge transport and recombination in dye-sensitized solar cells (DSSCs). Capillary stress created during evaporation of liquids from the mesopores of dense TiO2 NT arrays was of sufficient magnitude to induce bundling and microcrack formation. The average lateral deflection of the NTs in the bundles increased with the surface tension of the liquids and with the film thicknesses. The supercritical CO2 drying technique was used to produce bundle-free and crack-free NT films. Charge transport and recombination properties of sensitized films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. Transport became significantly faster with decreased clustering of the NTs, indicating that bundling creates additional pathways via intertube contacts. Removing such contacts alters the transport mechanism from a combination of one and three dimensions to the expected one dimension and shortens the electron-transport pathway. Reducing intertube contacts also resulted in a lower density of surface recombination centers by minimizing distortion-induced surface defects in bundled NTs. A causal connection between transport and recombination is observed. The dye coverage was greater in the more aligned NT arrays, suggesting that reducing intertube contacts increases the internal surface area of the films accessible to dye molecules. The solar conversion efficiency and photocurrent density were highest for DSSCs incorporating films with more aligned NT arrays owing to an enhanced light-harvesting efficiency. Removing structural disorder from other materials and devices consisting of nominally one-dimensional architectures (e.g., nanowire arrays) should produce similar effects.

High efficiency double heterojunction polymer photovoltaic cells using highly ordered TiO2 nanotube arrays

Vertically oriented TiO2 nanotube arrays formed by anodization offer a highly ordered material architecture for efficient charge generation and collection in photoelectrochemical devices. A blend of regioregular poly(3-hexylthiophene) and a methanofullerene (phenyl C71-butyric acid methyl ester) was infiltrated into transparent TiO2 nanotube films. The heterojunction poly(3-hexylthiophene) (P3HT)-([6,6]-phenyl-C71-butyric acid methyl ester) and P3HT-TiO2 interfaces both result in charge separation. The resulting solid state solar cells show a short-circuit current density of 12.4mA∕cm2, 641mV open circuit potential, and a 0.51 fill factor, yielding power conversion efficiencies of 4.1% under AM 1.5 sun.

Fabrication of Vertically Oriented TiO2 Nanotube Arrays Using Dimethyl Sulfoxide Electrolytes

We detail anodic oxidation variables affecting the fabrication of vertically oriented TiO2 nanotube arrays using an electrolyte of dimethyl sulfoxide (DMSO) containing either hydrofluoric acid (HF), potassium fluoride (KF), or ammonium fluoride (NH4F). Various anodization variables including F- ion concentration, voltage, anodization time, water content, and previous use of the electrolyte can be combined to achieve nanotube arrays with length and morphology relevant to required applications. Using an anodization potential of 60 V with an electrolyte of 2% HF in DMSO, 70 h duration, nanotubes are achieved having a length of 101 μm, inner diameter 150 nm, and wall thickness 15 nm for a calculated geometric area of 3475. The weak adhesion of the DMSO fabricated nanotubes to the underlying oxide barrier layer and low tube-to-tube adhesion facilitates their separation for applications where dispersed nanotubes are desired. We examine the photoelectrochemical properties of 45 μm long nanotube arrays, crystallized by annealing at 580 °C for 6 h in oxygen gas, tested under UV (320−400 nm) and solar simulated light (AM 1.5) illumination.

Thickness-conversion ratio from titanium to TiO2 nanotube fabricated by anodization method

Vertically oriented TiO2 nanotube arrays were fabricated by the anodization of pure titanium deposited on silicon substrates. The TiO2 nanotubes were grown by anodization in aqueous-base and glycerol-base electrolytes at different potentials between 5 V and 30 V. Films with homogeneously distributed pores having an average diameter of 100 nm or 40 nm were obtained. The dependences of pore morphology and pore formation rate on process parameters were evaluated. In particular, thickness-conversion ratio from titanium to TiO2 nanotube was presented to compare a growing character in different electrolytes.

Nanosize and Vitality: TiO2 Nanotube Diameter Directs Cell Fate

We generated, on titanium surfaces, self-assembled layers of vertically oriented TiO2 nanotubes with defined diameters between 15 and 100 nm and show that adhesion, spreading, growth, and differentiation of mesenchymal stem cells are critically dependent on the tube diameter. A spacing less than 30 nm with a maximum at 15 nm provided an effective length scale for accelerated integrin clustering/focal contact formation and strongly enhances cellular activities compared to smooth TiO2 surfaces. Cell adhesion and spreading were severely impaired on nanotube layers with a tube diameter larger than 50 nm, resulting in dramatically reduced cellular activity and a high extent of programmed cell death. Thus, on a TiO2 nanotube surface, a lateral spacing geometry with openings of 30-50 nm represents a critical borderline for cell fate.