Self-organized TiO2 Nanotube Arrays Research Articles

Synthesis of self-organized TiO2 nanotube arrays: Microstructural, stereoscopic, and topographic studies

In this work, titanium dioxide nanotubes were prepared by using titanium foils via electrochemical anodization in ethylene glycol solutions containing different amounts of water and fluoride in the ranges of 1%–3% and 0.15%–0.5%, respectively, to determine their effects on morphology, optical, and crystalline structure properties. Annealing processes were performed on all samples in the range between 273 and 723 K. Morphology and structure properties of the samples were studied by scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy. Titanium dioxide (TiO2) nanotubes, through anodization method, are strongly influenced by conditions, like fluoride concentration and applied voltages. Tube lengths between 2 and 7 μm were obtained, exhibiting different diameters and wall thicknesses. When alternating voltage was applied, the outer surface of the nanotubes exhibited evenly spaced ring-shaped regions, while smooth tubes were observed when constant voltage was applied. Reflection peaks, corresponding to Brookite, Anatase, and Rutile, of TiO2 phases, were observed from the XRD pattern. These phases were corroborated via μXRD measurements, and the Ti3O5 phase was also observed in detail. Absorption coefficient (α), optical band gap (Eg), and extinction coefficient (ε) of TiO2 nanotubes were calculated by transmittance spectra in the UV–Vis range. Strong absorption was noted in the UV region from reflectance and absorbance measurements. A correlation between synthesis parameters and physical properties is presented.

Controlled spacing of self-organized anodic TiO2 nanotubes

We report on how to grow and control self-organized TiO2 nanotube arrays that show defined and regular gaps between individual nanotubes. For this we use electrochemical anodization of titanium in fluoride containing di-ethylene glycol (DEG) electrolytes, with variations in voltage and water content in the electrolyte. In these specific electrolytes, such nanotubes show a true spacing, i.e. nanotubes are spaced both at top and at bottom in regular intervals, this in contrast to classic nanotubes obtained in other organic electrolytes showing a close-packed organization. We identify critical parameters that define the “region of existence” i.e. under which condition tube spacing occurs as well as the intertube distance, to be the voltage and the water content. Using these findings allows to grow tubes where diameter and spacing can even be independently controlled.

Fabrication of Smart Au/TiO2 Nanotubes/Si Based Schottky-Tunneling Diode Sensors for Electrochemical Detection of Biomarkers

Rapid, point-of-care (POC) disease diagnostics presents several challenges including cost, detection time, equipment portability, and performance. Detection of volatile organic compounds (VOCs) based disease biomarkers from the breath provides a potential solution to the aforementioned problems. One such tuberculosis (TB) biomarker identified from breath is methyl nicotinate. In this regard, development of smart sensor systems can provide an economically feasible solution and pave the way for rapid disease diagnostics. In this paper, we present the fabrication of a smart sensor developed by synthesis of self-organized TiO2 nanotube arrays formed by anodization of thin Ti film deposited on Si wafers by direct current (D.C.) magnetron sputtering. Site-specific interdigitated patterned growth of gold electrode was achieved using photolithography techniques. The gold interdigitated electrodes behave as the working electrode while platinum deposited on the back surface of the Si wafer acts as the back contact. Au on TiO2 presents a Schottky junction due to differences in their work function and can be overcome by light irradiation. On the other hand, an ultra-thin, inherent SiO2 layer on Si aids in tunneling electrons across the TiO2 – Si junction. Thus, Au/TiO2/SiO2/Si behaves as a Schottky tunneling diode junction that electronically couples a VOC oxidation catalyst (Au) to a photovoltaic absorber (Si). A sweep of the bias voltage (-5V to 5V) and associated sensor response at room temperature upon exposure to both air and biomarker vapors (~1000 ppm) was carried out. From the current-voltage plot (I-V), the operating voltage was chosen overcoming the negative differential resistance such that the sensor current response to the biomarker vapor would be maximized under forward bias condition. The sensor current when exposed to the biomarker vapor was higher than that in air. All sensor measurements were carried out under illuminated conditions. The variation in the output current of the sensor biased at the chosen operating voltage (~ 0.7V) at room temperature was observed for the range of 0.01 ppm to 1000 ppm biomarker concentration. The sensor demonstrated a strong response and the results suggest that the sensor is capable of detecting extremely small concentrations of the biomarker in the vapor phase. This smart sensor presents significant enhancement in sensor response over previously reported metal functionalized TiO2 nanotube array based sensing platform developed in our group for electrochemical detection of similar disease biomarkers.

Anodic TiO2 Nanotube Arrays as Fixed Photocatalyst

In the present study, self-organized TiO2 nanotube arrays were prepared by anodic oxidation in a viscous electrolyte of ethylene glycol containing NH4F with a concentration of 1,25% wt. the anodized samples expose single-walled nanotube arrays with an average pore diameter and wall thickness of about 80 and 15 nm, respectively. The photocatalytic efficiency of the nanotube arrays has been investigated and compared to commercial P25 nanopowder by measuring the degradation of methyl orange. The results showed that the nanotube arrays exhibit higher activity towards dye degradation as compared to commercial P25 nanoparticles with a degradation efficiency reaching up to 99.45% after an irradiation time of 60 min. The high surface to volume ratio and the mechanical adhesion makes them suitable as multiple use photocatalysts.

Ultrasmall graphitic carbon nitride quantum dots decorated self-organized TiO2 nanotube arrays with highly efficient photoelectrochemical activity

In the present study, a novel graphitic carbon nitride quantum dots (CNQDs) modified TiO2 nanotube arrays (NTAs) photoelectrode was successfully synthesized by a simple two-step method which includes an electrochemical anodization technique followed by a facile organic molecular linkage. The successful modification of TiO2 by CNQDs was found to improve the photoelectrochemical activity significantly because of enhanced light absorption and improved separation of photo-generated electron–hole pairs. The optimized CNQDs/TiO2 NTAs showed nearly 3.5-fold photocurrent of that from TiO2 NTAs alone at 0.3V vs Ag/AgCl and the maximum photoconversion efficiency reaches up to 0.63% in neutral solution under simulated solar light irradiation. Furthermore, the prepared CNQD/TiO2 NTAs demonstrated superior photoelectrocatalytic activity and stability in the degradation of RhB, with kinetic constants 3.0 times of that from TiO2 NTAs. The hydroxyl and superoxide radicals were proved to be the dominant active radicals during pollutants degradation. Additionally, the prepared CNQD/TiO2 sample was used as photoanodes for direct photoelectrochemical water splitting to produce H2 and O2 under simulated solar light illumination, achieving a H2 production rate with 22.0μmolh−1cm−2 at 0.3V vs Ag/AgCl.

Templated dewetting: designing entirely self-organized platforms for photocatalysis.

Formation and dispersion of metal nanoparticles on oxide surfaces in site-specific or even arrayed configuration are key in various technological processes such as catalysis, photonics, electrochemistry and for fabricating electrodes, sensors, memory devices, and magnetic, optical, and plasmonic platforms. A crucial aspect towards an efficient performance of many of these metal/metal oxide arrangements is a reliable fabrication approach. Since the early works on graphoepitaxy in the 70s, solid state dewetting of metal films on patterned surfaces has been much explored and regarded as a most effective tool to form defined arrays of ordered metal particles on a desired substrate. While templated dewetting has been studied in detail, particularly from a mechanistic perspective on lithographically patterned Si surfaces, the resulting outstanding potential of its applications on metal oxide semiconductors, such as titania, has received only limited attention. In this perspective we illustrate how dewetting and particularly templated dewetting can be used to fabricate highly efficient metal/TiO2 photocatalyst assemblies e.g. for green hydrogen evolution. A remarkable advantage is that the synthesis of such photocatalysts is completely based on self-ordering principles: anodic self-organized TiO2 nanotube arrays that self-align to a highest degree of hexagonal ordering are an ideal topographical substrate for a second self-ordering process, that is, templated-dewetting of sputter-deposited metal thin films. The controllable metal/semiconductor coupling delivers intriguing features and functionalities. We review concepts inherent to dewetting and particularly templated dewetting, and outline a series of effective tools that can be synergistically interlaced to reach fine control with nanoscopic precision over the resulting metal/TiO2 structures (in terms of e.g. high ordering, size distribution, site specific placement, alloy formation) to maximize their photocatalytic efficiency. These processes are easy to scale up and have a high throughput and great potential to be applied to fabricate not only (photo)catalytic materials but also a large palette of other functional nanostructured elements and devices.

Supercapacitive performance of hydrogenated TiO2nanotube arrays decorated with nickel oxide nanoparticles

Highly ordered self-organized TiO2 nanotube arrays (TNTAs) could not only be used as current collectors, but also adopted as highly ion-accessible and charge transfer-channels for the construction of supercapacitors. In this paper, hydrogenated TNTAs were obtained and then nickel oxide (NiOx) nanoparticles were successfully deposited onto the inner surface and interface of HTNTAs through a cyclic voltammetry electrochemical deposition process (NiOx/HTNTAs). The FESEM images of the samples showed that the diameter of the NiOx nanoparticles ranged from 7 to 60 nm. The as-fabricated NiOx/HTNTAs exhibited an obviously pseudocapacitive performance with a specific capacitance of 689.28 F g−1 at a current density of 1.5 A g−1 and 91.9% of the initial capacitance remaining after 5000 charge/discharge cycles at a current density of 3 A g−1 in 1 M KOH. This work reveals a feasible and green method for the fabrication of TNTAs modified with electroactive metal oxide nanoparticles as functional electrode materials for supercapacitors.

TiO2 nanotubes with laterally spaced ordering enable optimized hierarchical structures with significantly enhanced photocatalytic H2 generation.

In the present work we grow self-organized TiO2 nanotube arrays with a defined and controlled regular spacing between individual nanotubes. These defined intertube gaps allow one to build up hierarchical 1D-branched structures, conformally coated on the nanotube walls using a layer by layer nanoparticle TiO2 decoration of the individual tubes, i.e. having not only a high control over the TiO2 nanotube host structure but also on the harvesting layers. After optimizing the intertube spacing, we build host-guest arrays that show a drastically enhanced performance in photocatalytic H2 generation, compared to any arrangement of conventional TiO2 nanotubes or conventional TiO2 nanoparticle layers. We show this beneficial effect to be due to a combination of an increased large surface area (mainly provided by the nanoparticle layers) with a fast transport of the harvested charge within the passivated 1D nanotubes. We anticipate that this type of hierarchical structures based on TiO2 nanotubes with adjustable spacing will find even wider application, as they provide an unprecedented controllable combination of surface area and carrier transport.

Self-organized TiO2 nanotube arrays sensitized by uniform Cd0.8Zn0.2S nanoparticles for highly effective photoelectrochemical performance

TiO2 nanotube arrays sensitized by Cd0.8Zn0.2S nanoparticles (TiO2 NTs/CZS) were successfully fabricated by a two-step process of anodization followed by a hydrothermal method. Phase compositions and morphologies of TiO2 NTs/CZS were characterized by XRD and SEM, and the sensitization significantly improved the visible light response of TiO2 NTs, covering the UV and visible light with wavelengths below 550nm. TiO2 NTs/CZS showed high visible light photocurrent densities, which was attributed to the high visible light harvesting and the effective charge carrier separation derived from the coupling effect of TiO2 NTs and CZS. TiO2 NTs/CZS also showed high photoelectrocatalytic properties, including decomposition of methyl orange (MO) and reduction of Cr(VI) into Cr(III).

Electrodeposition Synthesis of Polyaniline-Modified TiO2 Nanotube Arrays with Enhanced Photoelectrochemical Property

Polyaniline-modified TiO2 nanotube arrays (PANI/TNTs) were fabricated by a constant voltage electropolymerization method. The self-organized TiO2 nanotube arrays with an inner diameter of approximately 100 nm and a length of 4 μm were prepared by a two-step anodization at 20 V for 3 h, and then the polyaniline was deposited on the surface of the TiO2 nanotube arrays. The phase composition, morphology and property of PANI/TNTs were characterized by X-ray diffraction, scanning electron microscopy, fourier transform infrared spectroscopy and UV-vis diffuse reflectance spectroscopy. The prepared PANI/TNTs showed enhanced visible light absorption and corresponding visible light photocatalytic activity. Furthermore, the PANI/TNTs also exhibited higher electrochemical behaviour than that of TiO2 nanotube arrays.

Self-organized TiO2 nanotube arrays in the photocatalytic degradation of methylene blue under UV light irradiation

Nanostructured titanium dioxide (NTiO2) is known to possess efficient photocatalytic activity and to have diverse applications in many fields due to its chemical stability, high surface area/volume ratio, high transmittance, and high refractive index in the visible and the near-ultraviolet regions. These facts prompted us to develop TiO2 nanotube (TiO2 NT) arrays through electrochemical anodic oxidation involving different electrolytes comprised of phosphoric acid — hydrofluoric acid aqueous systems by varying the voltage and the time. The annealing temperature of the nanotubes, TiO2 NTs, were varied to modify the surface morphology and were characterized by using X-ray diffraction and scanning electron microscopy. Scanning electron microscopy and X-ray diffraction results showed that the samples had uniform morphologies and good crystalline structures of the anatase phase at lower annealing temperatures and of the rutile phase at higher annealing temperatures. A secondary-ion mass-spectrometry analysis was used to investigate the surface atoms and to conduct a depth profile analysis of the TiO2 NTs. The efficiency of the photocatalytic activity of the TiO2 NT arrays in degrading methylene blue (MB) was investigated under UV-Vis light irradiation. The maximum photocatalytic activity was achieved for the samples with lower annealing temperatures due to their being in the anatase phase and having a higher surface area and a smaller crystal size, which play important roles in the degradation of organic pollutants.

GISAXS and TOF-GISANS studies on surface and depth morphology of self-organized TiO2nanotube arrays: model anode material in Li-ion batteries

Self-organized anodic titania (TiO2) nanotube arrays are an interesting model anode material for use in Li-ion batteries owing to their excellent rate capability, their cycling stability and their enhanced safety compared to graphite. A composite material where carbothermally treated conductive TiO2nanotubes are used as support for a thin silicon film has been shown to have the additional advantage of high lithium storage capacity. This article presents a detailed comparison of the structure, surface and bulk morphology of self-organized conductive TiO2nanotube arrays, with and without silicon coating, using a combination of X-ray diffraction, X-ray reflectivity, grazing-incidence small-angle X-ray scattering (GISAXS) and time-of-flight grazing-incidence small-angle neutron scattering (TOF-GISANS) techniques. X-ray diffraction shows that the nanotubes crystallize in the anatase structure with a preferred (004) orientation. GISAXS and TOF-GISANS are used to study the morphology of the nanotube arrays, delivering values for the inner nanotube radius and intertubular distances with high statistical relevance because of the large probed volume. The analyses reveal the distinct signatures of a prominent lateral correlation of the TiO2nanotubes of ∼94 nm and a nanotube radius of ∼46 nm. The porosity averaged over the entire film using TOF-GISANS is 46%. The inner nanotube radius is reduced to half (∼23 nm) through the silicon coating, but the prominent lateral structure is preserved. Such in-depth morphological investigations over large sample volumes are useful towards development of more efficient battery electrode morphologies.

Electrochemical behavior and effect of heat treatment on morphology, crystalline structure of self-organized TiO2 nanotube arrays on Ti-6Al-7Nb for biomedical applications.

In the present work, we investigate the formation of self-organized titanium oxide nanotube layers by anodic oxidation on titanium alloy Ti-6Al-7Nb in electrolyte solution containing sulfuric acid and hydrofluoric acid. The anodized surface was characterized by micro-Raman, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDS). The corrosion behavior of the treated and untreated samples was investigated through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization studies in simulated body fluid (Hanks' solution). The investigations show that the native oxide on the sample is replaced by self-assembled nanoarray by anodization. FESEM of samples annealed at 450 to 800 °C show tubular morphology whereas those annealed at 850 °C show collapse of nanotubes. Electrochemical impedance data of the substrate and 10 V anodized samples were fitted with a two-time constant equivalent circuit and that of anodized samples (20, 30 V) with a three-time constant equivalent circuit.

Effect of NH4F concentration and controlled-charge consumption on the photocatalytic hydrogen generation of TiO2 nanotube arrays

Self-organized TiO2 nanotube arrays (TiO2 Nts) for hydrogen evolution were successfully fabricated by electrochemical anodization in ethylene glycol-based electrolytes with various NH4F concentrations. Hydrogen production by photocatalytic water splitting was performed in a two-compartment photoelectrochemical (PEC) cell without any applied voltage. The effect of NH4F concentration on the morphological structure, PEC performance and hydrogen evolution of TiO2 samples were systematically investigated. TiO2 anodized with 0.50wt% of NH4F concentration for 60min exhibited highest hydrogen evolution of 2.53mLh−1cm−2 and maximum photoconversion efficiency of 4.39%. Based on the above results and the different current-time curves during anodization processes, another series of TiO2 nanotube arrays samples with the equal charge consumption (designated as TiO2 Nts-EC) were synthesized by controlling the anodization time in electrolytes containing different NH4F concentrations. Compared with TiO2 Nts prepared at the same NH4F concentration for 60min, hydrogen production and PEC properties of TiO2 Nts-EC samples have been obviously enhanced, especially for the sample prepared at 1.25wt% NH4F concentrations. Moreover, despite the difference of NH4F concentration in electrolyte, the TiO2 Nts-EC samples exhibit comparable PEC performances. These results indicated that the anodization charge-density play a crucial role in the hydrogen generation of TiO2 Nts. The results benefit the enhancement of the hydrogen production of TiO2 Nts.

Electrochemical behaviour and bioactivity of self-organized TiO2 nanotube arrays on Ti-6Al-4V in Hanks’ solution for biomedical applications

In the present work, the formation of self-organized titanium oxide nanotube layer by anodic oxidation on titanium alloy Ti-6Al-4V and its influence on growth of apatite on Ti-6Al-4V is reported. Anodization was carried out in sulfuric acid and hydrofluoric acid containing electrolyte and the anodized surface was characterized by micro-Raman, XRD and FESEM. The corrosion behavior of the anodized sample was investigated through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization studies in simulated body fluid (Hanks’ solution). The influence of TiO2 nano tubes on the growth of apatite on the anodized sample was investigated by immersion in Hanks’ solution for 1 and 7 days. The investigations show that the native oxide on the sample is replaced by self assembled nanotube array by anodization. The anodic oxide consist both anatase and rutile. After vacuum annealing at 600°C, intensity of rutile peak increased. FESEM of samples annealed at 450°C show tubular morphology whereas those annealed at 600°C show collapse of nano tubes. Corrosion resistance of the anodized sample was comparable with that of the untreated samples. Electrochemical impedance data of the substrate was fitted with two time constant equivalent circuit and that of anodized samples with three time constant equivalent circuit. FESEM images of immersed samples show that the growth of apatite is more with larger sized deposits on anodized surface as compared to that on the untreated substrate. XPS investigation of immersed sample shows presences of calcium, phosphorous and oxygen in hydroxide/phosphate form on the anodized sample.

Controlled synthesis of Cu nanoparticles on TiO2 nanotube array photoelectrodes and their photoelectrochemical properties

TiO2 NTs/Cu photoelectrodes were prepared by coating Cu nanoparticles on the tube wall of the self-organized TiO2 nanotube arrays using a successive ionic layer adsorption and reaction method. The structural investigation by X-ray diffraction and scanning electron microscopy indicated that Cu nanoparticles grew uniformly on the walls of TiO2 nanotubes. The modification of Cu nanoparticles improved the photoresponse and photocurrent density of TiO2 NTs into the visible light region.

Enhanced Visible-Photocatalytic Activity of Anodic TiO2 Nanotubes Film via Decoration with CuInSe2 Nanocrystals

The fabrication and photocatalytic properties of visible-light driven CuInSe2/TiO2 heterojunction films are reported. CuInSe2 nanoparticles (NCs) were synthesized using a solvothermal method and then decorated onto self-organized anodic TiO2 nanotube (NT) arrays through an electrophoretic deposition process, forming a CuInSe2 NC/TiO2 NT hetero-structure film. An increase in deposition time produced an increased amount of CuInSe2 NCs loaded onto the TiO2 NT arrays, expanding the light-absorption range of the CuInSe2 NCs/TiO2 NTs film from 400 nm to 700 nm. Photocatalytic degradation results show that activities of the CuInSe2 NCs/TiO2 NTs films were significantly enhanced compared to that of pure TiO2 NTs (degradation rate constant k increased from 3 × 10(-3) min(-1) to >1 × 10(-2) min(-1)). Particularly, the CuInSe2 NCs/TiO2 NTs with 50 min electrophoretic deposition show the highest degradation rate, k = 1.6 × 10(-2) min(-1) (more than 5 times greater than that of the pure TiO2 NTs film), due to optimization of CuInSe2 NCs loading and a well-maintained open TiO2 tube-mouth configuration.

Synthesis and Growth Mechanism of Thin-Film TiO2 Nanotube Arrays on Focused-Ion-Beam Micropatterned 3D Isolated Regions of Titanium on Silicon

In this paper, the fabrication and growth mechanism of net-shaped micropatterned self-organized thin-film TiO2 nanotube (TFTN) arrays on a silicon substrate are reported. Electrochemical anodization is used to grow the nanotubes from thin-film titanium sputtered on a silicon substrate with an average diameter of ~30 nm and a length of ~1.5 μm using aqueous and organic-based types of electrolytes. The fabrication and growth mechanism of TFTN arrays from micropatterned three-dimensional isolated islands of sputtered titanium on a silicon substrate is demonstrated for the first time using focused-ion-beam (FIB) technique. This work demonstrates the use of the FIB technique as a simple, high-resolution, and maskless method for high-aspect-ratio etching for the creation of isolated islands and shows great promise toward the use of the proposed approach for the development of metal oxide nanostructured devices and their integration with micro- and nanosystems within silicon-based integrated-circuit devices.

Passivated n–p co-doping of niobium and nitrogen into self-organized TiO2 nanotube arrays for enhanced visible light photocatalytic performance

Passivated n–p co-doping of niobium and nitrogen was successfully incorporated into self-organized TiO2 nanotube arrays by anodizing Ti–Nb alloys, followed with the heat treatment in a flow of ammonia gas. Nb was doped into TiO2 nanotube arrays during the anodization by substituting Ti4+ with Nb5+, while N was doped into TiO2 nanotube arrays during the heat treatment by substituting O2− with N3−. Since Nb in TiO2 enhanced the adsorption of NH3 molecules onto the nanotube arrays, Nb doping was found to promote the subsequent N doping into the anatase lattice. As predicted by first-principles band structure calculations, Nb/N co-doped titanium oxide nanotube arrays demonstrated a largely enhanced visible light response and visible light photocatalytic performance on the degradation of methylene blue, compared to TiO2 nanotube arrays or TiO2 nanotube arrays with either dopant. The passivated n–p co-doping approach may also be applied to other material systems and promise a wide range of technical applications.

Fabrication of self-organized TiO2 nanotube arrays for photocatalytic reduction of CO2

The photocatalytic conversion of CO2 and H2O to alcohols was achieved using self-organized TiO2 nanotube arrays (TNAs), which were prepared by electrochemical anodization of Ti foils in 1 M (NH4)2SO4 electrolyte containing 0.5 wt% NH4F. Experimental results revealed that the morphology and structure of self-organized TNAs could be strongly influenced by the applied voltage and anodization temperature, and the optimized TNAs were prepared by electrochemical anodization of Ti foils under optimal conditions (i.e., at 20 V for 2 h at 30 °C). The as-prepared TNAs were amorphous and could be transformed to anatase phase during the thermal treatment at 450 °C in air for 3 h. By using the annealed TNAs as a photocatalyst, the photocatalytic reduction of CO2 to alcohol, predominately methanol and ethanol, was demonstrated under Xenon lamp illumination. Based on the photocatalytic measurements, the production rates of methanol and ethanol were calculated to be ∼10 and ∼9 nmol cm−2 h−1, respectively. In addition, the formation mechanism of methanol and ethanol was also tentatively proposed.