Self-organized TiO2 Nanotube Arrays Research Articles

Transparent Anodic TiO<SUB>2</SUB> Nanotube Arrays on Plastic Substrates for Disposable Biosensors and Flexible Electronics

Exploitation of anodically formed self-organized TiO2 nanotube arrays in mass-manufactured, disposable biosensors, rollable electrochromic displays and flexible large-area solar cells would greatly benefit from integration with transparent and flexible polymeric substrates. Such integration requires the vacuum deposition of a thin film of titanium on the desired substrate, which is then anodized in suitable media to generate TiO2 nanotube arrays. However the challenges associated with control of Ti film morphology, nanotube array synthesis conditions, and film adhesion and transparency, have necessitated the use of substrate heating during deposition to temperatures of at least 300 degrees C and as high as 500 degrees C to generate highly ordered open-pore nanotube arrays, thus preventing the use of polymeric substrates. We report on a film growth technique that exploits atomic peening to achieve high quality transparent TiO2 nanotube arrays with lengths up to 5.1 microm at room temperature on polyimide substrates without the need for substrate heating or substrate biasing or a Kauffman ion source. The superior optical quality and uniformity of the nanotube arrays was evidenced by the high specular reflectivity and the smooth pattern of periodic interferometric fringes in the transmission spectra of the nanotube arrays, from which the wavelength-dependent effective refractive index was extracted for the air-TiO2 composite medium. A fluorescent immunoassay biosensor constructed using 5.1 microm-long transparent titania nanotube arrays (TTNAs) grown on Kapton substrates detected human cardiac troponin I at a concentration of 0.1 microg ml(-1).

Self-organized amorphous TiO2 nanotube arrays on porous Ti foam for rechargeable lithium and sodium ion batteries

Self-organized amorphous TiO2 nanotube arrays (NTAs) were successfully fabricated on both Ti foil and porous Ti foam through electrochemical anodization techniques. The starting Ti foams were fabricated using ARCAM's Electron Beam Melting (EBM) technology. The TiO2 NTAs on Ti foam were used as anodes in lithium ion batteries; they exhibited high capacities of 103 μAh cm−2 at 10 μA cm−2 and 83 μAh cm−2 at 500 μA cm−2, which are two to three times higher than those achieved on the standard Ti foil, which is around 40 μAh cm−2 at 10 μA cm−2 and 24 μAh cm−2 at 500 μA cm−2, respectively. This improvement is mainly attributed to higher surface area of the Ti foam and higher porosity of the nanotube arrays layer grown on the Ti foam. In addition, a Na-ion half-cell composed of these NTAs anodes and Na metal as the counter electrode showed a self-improved specific capacity upon cycling at 10 μA cm−2. These results indicate that TiO2 NTAs grown on Ti porous foam are promising electrodes for Li-ion or Na-ion rechargeable batteries.

Fabrication of TiO2 nanotube–nanocube array composite electrode for dye-sensitized solar cells

One dimensional TiO2 nanotube structure recently plays an important role in the application of dye sensitized solar cells (DSSCs) due to its faster electron transport. The fabrication of photoanode using the TiO2 nanotube structures mixed with the TiO2 nanoparticles was investigated to enhance the photovoltaic efficiency of DSSCs by increasing the surface area of electrode. In this work, self-organized and vertically-oriented TiO2 nanotube arrays (TNAs) covered with uniformly distributed TiO2 nanocubes (TNCs) were fabricated in a simple one-step anodization process. The X-ray diffraction patterns reveal that both TNAs and TNCs are in anatase phase. The scanning electron microscopy analysis demonstrates that the wall thickness and inner diameter of hexagonal close-packed TiO2 nanotubes from chemically polished Ti foils are 10–15 and 100–120nm, respectively, and the particle size of TNCs is 60–75nm. The DSSC fabricated by the mixed morphological TNAs with TNCs shows an enhanced photoconversion efficiency of ~63% than that of TNAs alone, due to the increase of both dye adsorption and electron transportation rate.

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.

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 30–120nm and length in 0.6–3μm. 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 2.2μm in length combining with N719 dye, and the corresponding photovoltaic parameters of 3.6mAcm−2 in short circuit photocurrent density, 840mV in open circuit potential, and 63.2% in fill factor.

Self-organized TiO2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting

Highly efficient water splitting electrode based on uniform platinum (Pt) nanoparticles on self-organized TiO2 nanotube arrays (TNTAs) was prepared by a combination of multi-step electrochemical anodization with facile photoreduction process. Uniform platinum (Pt) nanoparticles with an average diameter of 8 nm are distributed homogeneously on nanoporous top layer and underneath TiO2 nanotube wall. In comparison to pristine TNTAs, Pt@TNTAs show substantially enhanced photocurrent density and the incident photon-to-current conversion efficiency (IPCE) in the entire wavelength window. The maximum photocurrent density and IPCE from the optimized Pt@TNTAs photoelectrode (Pt, ～1.57 wt%) were about 24.2 mA cm−2 and 87.9% at 350 nm, which is much higher than that of the pure nanotubes sample (16.3 mA cm−2 and 67.3%). The resultant Pt@TNTAs architecture exhibited significantly enhanced photoelectrochemical activities for solar water splitting with hydrogen evolution rate up to 495 μmol h−1 cm−2 in 2 M Na2CO3 + 0.5 M ethylene glycol under the optimal external bias of −0.3 VSCE.

Nb-doping of TiO2/SrTiO3 nanotubular heterostructures for enhanced photocatalytic water splitting

Ti alloys with low Nb contents (0.1at.% and 0.5at.%) were used to grow self-organized Nb-doped TiO2 nanotube arrays. These layers were partially converted to SrTiO3 using a hydrothermal treatment. Various factors, such as hydrothermal temperature, hydrothermal duration, and annealing conditions, were investigated in view of the photocatalytic water splitting performance of the formed Nb-doped TiO2/SrTiO3 heterostructure nanotubes. Photoelectrochemical characterization shows that Nb-doping efficiently enhances the photocatalytic water splitting performance of TiO2/SrTiO3 heterostructure nanotubes.

Gradient TiO2 Nanotube Arrays via Asymmetric Anodization

Gradient self-organized TiO2 nanotube arrays with the tube diameters and length gradually changing along the sample plane are conveniently fabricated. The fabrication method is based on asymmetric anodization with the point cathode placed close to the edge of the titanium foil anode. Mechanism study reveals that the drastic variance in the voltage drop over the tube bottom at different locations along the film is responsible for the formation of the gradient morphology. Experimentally, moderate electrolyte conductivities and sufficiently high anodization voltages are necessary to enable the desired gradient TiO2 nanotube arrays. Moreover, the structural features of the fabricated gradient TiO2 nanotube arrays can be further adjusted by controlling the imperfection level of the titanium substrate.

Efficient visible light-induced photoelectrocatalytic degradation of rhodamine B by polyaniline-sensitized TiO2 nanotube arrays

Self-organized anodic TiO2 nanotube arrays were sensitized with polyaniline by a simple electrodeposite method. The morphological and structural properties studied by scanning electron microscopy and fourier transform infrared spectroscopy reveal the successful deposition of polyaniline on the nanotube arrays. The polyaniline-sensitized TiO2 nanotube arrays exhibit a distinguishable red shift on the absorption spectrum. Electrochemical impedance investigation attested to a significant improvement of the interfacial electron-transfer kinetics for promoted electron–hole effective separation. The as-prepared samples showed a high efficiency for the photoelectrocatalytic degradation of rhodamine B under visible-light irradiation (λ > 400 nm). The enhanced photoelectrocatalytic activity could be attributed to the extended absorption in the visible-light region by the polyaniline and the effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the polyaniline/TiO2 nanotube arrays interface.

A photoelectrochemical study of CdS modified TiO2 nanotube arrays as photoanodes for cathodic protection of stainless steel

An electrodeposited CdS nanoparticles-modified highly-ordered TiO2 nanotube arrays (CdS–TNs) photoelectrode and its performance of photocathodic protection are reported. The self-organized TiO2 nanotube arrays are fabricated by electrochemical anodization in an organic–inorganic mixed electrolyte and sensitized with CdS nanoparticles by electrodeposition via a single-step direct current. The morphology, crystalline phase, and composition of the CdS–TNs films were characterized systematically by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and ultraviolet–visible (UV–Vis) spectroscopy, respectively. The photoelectrochemical performances of the CdS–TNs film under illumination and dark conditions in 0.5M NaCl solution were evaluated through the electrochemical measurements. It is indicated that the TNs incorporated by CdS effectively harvest solar light in the UV as well as the visible light (up to 480nm) region. It is supposed that the high photoelectro-response activity of the CdS–TNs is attributed to the increased efficiency of charge separation and transport of electrons. The electrode potentials of 304 stainless steel coupled with the CdS–TNs is found to be negatively shifted for about 246mV and 215mV under UV and white light irradiation, respectively, which can be remained for 24h even in darkness. It is implied that the CdS–TNs are able to effectively function a photogenerated cathodic protection for metals both under the UV and visible light illumination.

Effect of Applied Potential on the Formation of Self-OrganizedTiO2Nanotube Arrays and Its Photoelectrochemical Response

Self-organized TiO2nanotube arrays have been fabricated by anodization of Ti foil in an electrochemical bath consisting of 1 M of glycerol with 0.5 wt% of NH4F. The effects of applied potential on the resulting nanotubes were illustrated. Among all of the applied potentials, 30 V resulted in the highest uniformity and aspect ratio TiO2nanotube arrays with the tube's length approximately 1 μm and pore's size of 85 nm. TiO2nanotube arrays were amorphous in as-anodized condition. The anatase phase was observed after annealing at 400∘Cin air atmosphere. The effect of crystallization and effective surface area of TiO2nanotube arrays in connection with the photoelectrochemical response was reported. Photoelectrochemical response under illumination was enhanced by using the annealed TiO2nanotube arrays which have larger effective surface area to promote more photoinduced electrons.

A brief review on fabrication and applications of auto-organized TiO2 nanotube arrays

Fabrication and applications of self-organized TiO2 nanotube arrays is briefly reviewed. This nanoscale material has attracted considerable attention owing to its potential in gas sensing, photo-thermal and photovoltaic conversion devices, photoelectrolysis and photocatalysis, etc. TiO2 in nanowire form, nanotube form and other nanoporous structures is desirable because of its high surface–volume ratio, high surface activity and superior sensitivity. TiO2 nanotubes and nanofibers have been developed by diverse techniques such as sol-gel, electrodeposition and anodization. In recent years, there has been significant progress in nano-structured TiO2 fabrication using the anodization method. This consists of oxidation (corrosion) of titanium foils or titanium thin films deposited onto substrates (e.g., silicon), immersed in acid or neutral F-containing solutions. Significant interest has arisen in anodization parameters, such as applied potential, concentration and type of corrosive solutions, anodization time length and temperature. This parameters influence mechanical-structural, chemical and electrical features of porous TiO2. Recent work concerning fabrication, applications and growth models is described in this review paper.

Preparation and Adsorption Properties of Pd Nanoparticles Supported on TiO2Nanotubes

Here we present preparation methods for a series of novel model heterogeneous catalysts consisting of Pd nanoparticles dispersed on self-organized TiO2 nanotube arrays (TiNTs). For a comprehensive comparative study of TiNTs’ surface adsorption properties, different chemical and physical methods were employed for the deposition of the Pd nanoparticles: (i) e-beam deposition in high vacuum (HV) and (ii) in ultrahigh vacuum (UHV), (iii) impregnation, and (iv) metal precipitation. Following preparation, the Pd/TiNTs were exposed to several O2 (oxidative) and CO (reductive) cleaning cycles at 500 K in UHV. After each cleaning cycle, the CO adsorption properties were systematically probed with time-resolved infrared reflection−adsorption spectroscopy to monitor the surface state of the catalyst. It is shown that clean and well-defined Pd/TiNT catalysts can be obtained, which are well suited for future adsorption and reactivity studies.

Self‐organized TiO2 Nanotube Arrays: Critical Effects on Morphology and Growth

AbstractTiO2 nanotube layers possess a wide range of applications for energy conversion, environmental clean‐up, and biomedical implant materials. The formation process of these tube layers is based on a simple but highly optimized anodization process of Ti in a suitable solution.The present work gives a coherent overview on the effect of the key parameters to grow these tube layers. The influence of fluoride content, anodization voltage, water content, and temperature on tube morphology is shown and discussed. The results thus provide the reader with “recipes” and understanding to grow desired TiO2 nanotube structures, as well as show current limits and open room for improvement for the tailored growth of these structures.

Sonication-Assisted Synthesis of CdS Quantum-Dot-Sensitized TiO2 Nanotube Arrays with Enhanced Photoelectrochemical and Photocatalytic Activity

A sonication-assisted sequential chemical bath deposition (S-CBD) approach is presented to uniformly decorate CdS quantum dots (QDs) on self-organized TiO2 nanotube arrays (TNTAs). This approach avoids the clogging of CdS QDs at the TiO2 nanotube mouth and promotes the deposition of CdS QDs into the nanotubes as well as on the tube walls. The photoelectrochemical and photocatalytic properties of the resulting CdS-decorated TNTAs were explored in detail. In comparison with a classical S-CBD approach, the sonication-assisted technique showed much enhancement in the photoelectrochemical and photocatalytic activities of the CdS QDs-sensitized TNTAs.

Structural Effects of Nanotubes, Nanowires, and Nanoporous Ti/TiO2 Electrodes on Photoelectrocatalytic Oxidation of 4,4-Oxydianiline

The present work illustrates the effect of electrolyte composition on the self-organized TiO2 nanotube arrays electrode preparation. The influence of structural and surface morphology of the TiO2 nanotube-like anode on their photoactivity and photoelectrocatalytic performance was also investigated. TiO2 nanotubular array electrodes are grown by anodization of Ti foil in 0.25wt % NH4F/ glycerol/water, but nanowires can be obtained in 4% HF-DMSO as supporting electrolyte, even when both are subjected to electrochemical anodization at 30 V during 50 h. The morphological characteristics are analyzed by X-ray diffraction (XRD) and field emission scanning electron microscope (FEG-SEM). The electrodes were successfully applied in photoelectrocatalytic oxidation of 4,4′- oxydianiline (ODAN) in aqueous solution, as a model of a harmful pollutant. Complete removal of the aromatic amine was obtained after 3 hours of photoelectrocatalytic treatment on nanotubular arrays electrodes.

A NiO/TiO2 junction electrode constructed using self-organized TiO2 nanotube arrays for highly efficient photoelectrocatalytic visible light activations

One dimensional (1D), self-organized TiO2 nanotube arrays are known to have excellent charge transport properties and a NiO/TiO2 junction is efficient in separating electron–hole pairs. This paper describes the synthesis of a NiO/TiO2 junction electrode constructed using self-organized TiO2 nanotube arrays combining the above two properties. The self-organized TiO2 nanotube arrays used in this study were prepared by anodizing titanium films, which resulted in closely packed n-type TiO2 tubes with an inner pore diameter of 60–90 nm, a wall thickness of approximately 15 nm and a length of 600 nm. The NiO/TiO2 junction was synthesized by electroless plating and annealing which resulted in TiO2 nanotube arrays coated with a layer (about 200 nm in thickness) of NiO particles (20–40 nm). The resulting NiO/TiO2 junction electrode enabled us to obtain an enhanced photocurrent (3.05 mA cm−2) as compared with a TiO2 electrode based on TiO2 nanotube arrays (0.92 mA cm−2) under AM 1.5 G (100 mw cm−2) at a bias of 0.65 V.

Kinetics and Mechanisms for Photoelectrochemical Degradation of Glucose on Highly Effective Self-Organized TiO2 Nanotube Arrays

The kinetics and mechanisms of photoelectrochemical catalytic degradation of glucose on self-organized TiO2 nanotube arrays (TNAs) were investigated. A thin-layer cell was used to obtain an exhausted reaction condition with which an overall degradation process of glucose could be identified including surface reaction on TNAs and mass transfer from body solution to the diffuse layer. Current-time (Iph-t) and the corresponding differential coefficient profiles were used to analyze the micro-processes of photoelectrochemical catalytic degradation. The initially generated photocurrents on glucose degradation versus glucose concentrations fits well with Langmuir adsorption isotherm, I0ph = 0.00008c0/(1+0.69274c0)+0.00034. This confirmed the adsorption of glucose on TNAs film catalyst was a single molecule layer adsorption, and the photoelectrochemical catalytic degradation reaction kinetics on TNAs surface belonged to a first-order reaction. After the initial quick reaction, three consecutive micro kinetic processes were revealed by the differential coefficient profiles (dIph/dt-t) of the glucose degradation profiles (Iph-t).

SnO2 nanocrystals on self-organized TiO2 nanotube array as three-dimensional electrode for lithium ion microbatteries

Three-dimensional (3D) anodes have been prepared for lithium ion microbatteries by depositing SnO2 nanocrystals into self-organized TiO2 nanotube arrays through the solvothermal method, with the SnO2 crystal size less than 5 nm. The 3D SnO2–TiO2 anodes exhibit excellent electrochemical performance with a good capacity retention of up to 70.8% over 100 cycles in the voltage range of 0.05–2.5 V. SnO2 enhances the capacity to more than double that of bare TiO2, while TiO2 nanotubes accommodate the volume changes of SnO2 during charge/discharge cycling. The amount of SnO2 loading can be controlled by varying the reaction time. The capacity of the 3D electrodes is controlled by the TiO2 tube length as well as by the amount of SnO2 loading. The maximum reversible capacity of the present samples can reach as high as about 300 µA h cm−2.

Amorphous and Crystalline TiO2 Nanotube Arrays for Enhanced Li-ion Intercalation Properties

AbstractWe have employed anodic oxidation of Ti foils to prepare self-organized TiO2 nanotube arrays which show enhanced electrochemical properties for applications as Li-ion battery electrode materials. The lengths and pore diameters of TiO2 nanotubes can be finely tuned by varying voltage, electrolyte composition, or anodization time. The as-prepared nanotubes are amorphous and can be converted into anatase nanotubes with heat treatment at 480oC and nanotubes of mixed anatase/rutile phases by heating at 580oC. The morphological features of nanotubes remain unchanged after annealing. Amorphous nanotubes with a length of 3.0 μm and an outer diameter of 125 nm delivers a capacity of 91.2 μA h cm-2 at a current density of 400 μA cm-2, while those with a length of 25 μm and an outer diameter of 158 nm display a capacity of 533 μA h cm-2. The 3-μm long anatase nanotubes and nanotubes of mixed phases show lower capacities of 53.8 μA h cm-2 and 63.1 μA h cm-2, respectively at the same current density. The amorphous TiO2 nanotubes with a length of 1.9 μm exhibit a capacity five times higher than that of TiO2 compact layer even when the nanotube array is cycled at a current density 80 times higher than that for the compact layer. The amorphous nanotubes show excellent capacity retention ability over 50 cycles. Cycled nanotubes show little change in morphology compared to the nanotubes before cycling, indicating the high structural stability of TiO2 nanotubes.