Anodic Growth Of TiO2 Nanotubes Research Articles

Anodic growth of TiO2 nanotube arrays: Effects of substrate curvature and residual stress

We study anodic growth of TiO2 nanotubes on the surfaces of Ti rings of different thicknesses in a two-electrode cell under 50 V with an anodization time of 30 min at room temperature. The experimental results demonstrate that there are three factors controlling the growth of TiO2 nanotubes on the surfaces of Ti rings – ring radius, ring thickness and residual stress. The length of TiO2 nanotubes decreases with the increase of the radius of Ti rings and the decrease of the ring thickness. The residual stress hinders the growth of TiO2 nanotubes on the surfaces of Ti rings.

Free-standing TiO2 nanograssy tubular hybrid membrane for polysulfide trapping in Li-S battery.

During the growth of anodic TiO2 nanotubes with a high layer thickness of greater than 20 μm, "nanograss" structures are typically formed on the outermost surface. This happens due to the fact that the engraving of the oxide tubes arises during prolonged exposure to an F- ion containing electrolyte. These TiO2 nanotubular layers have a high aspect ratio with astonishing bundles of nanograss structures on the tube top and especially a high surface area with anatase crystallites in the tubes. By two-step anodization in synergy with the hybridization of a rubber polymer binder, freestanding nanotubular layers consisting of nanograssy surfaces with nano-crystalline particles in the tubes were successfully obtained. Under the highly efficient polysulfide trapping and electrolyte perturbation, this nanotubular hybrid membrane could deliver an enriched performance with a capacity of 618 mA h g-1 after 100 cycles at 0.1C in Li-S batteries.

Effect of Dense Oxide Film Thickness on the Growth of Lower Layer Nanotubes

There are two opposite mechanisms for the growth of anodic TiO2 nanotubes. One is the field-assisted dissolution and ejection theory, the other is the oxygen bubble mold. In order to prove the latter mechanism is right, we constructed a three-layer nanotube structure (upper layer nanotubes + dense oxide film + lower layer nanotubes), which was obtained by three steps anodization. The upper and lower layers nanotubes are separated by a dense oxide film. When the dense oxide film is thin, the lower layer nanotubes is longer than the upper layer nanotubes. On the contrary, when the dense oxide film is thick, the lower layer nanotubes is shorter than the upper layer nanotubes. In either case, the anodization conditions of the upper and lower layer nanotubes are the same. This could not be explained by field-assisted dissolution theory. By analyzing the curve, we introduce a new method to decide the time of entering the third stage of the anodization, which is the main stage of nanotubes growth. With the second anodization voltage increases, the time edge entering the third stage become longer, which means the time to growth the nanotubes becomes shorter, and finally leading the different length of the nanotubes.

Controlling anodization time to monitor film thickness, phase composition and crystal orientation during anodic growth of TiO2 nanotubes

Anodic TiO2 nanotube (TNT) films show promises for photon-driven catalytic, electricity storage and chemical processes. The film thickness of anodic TNT is known to affect its performance in optical and electronic applications. Also, factors affecting the morphology and dimensions of anodic TNT films are rather well-known. However, the knowledge on phase transition and composition in the growth of anodic TiO2 from the titanium metal is very limited. In this work, the anodization time is controlled in intervals of 10, 60, 300, 1000, 2000 and 5000 s to investigate its effect on phase composition and transition, and the morphology of the anodic TNT during the growth process. Even though the mechanism of anodic TNT formation is still under debate, the scanning electron microscope results support bottom-up tube growth with evidence of a compact layer. It was also found that the Richards growth model is applicable to correlate growth time and film thickness. Finally, the phase transition, crystal orientation and pore formation during the anodic process are further discussed.

Current density transient response to variations in synthesis parameters during anodic growth of TiO2 nanotubes

The field of TiO2 photo-electrode synthesis is fast-growing, due to the competitive performance of TiO2 in light-harvesting applications such as photo-electrochemical water splitting. Real-time monitoring of TiO2 nanotube array (NTA) synthesis is critical for reproducibility in a production chain of identical photo-electrodes. By analysing the transient stage of a current density signal, this work demonstrates the expected responses to changes in anodic voltage and electrolyte content. Seven titanium foil substrates were anodized in an anhydrous polar organic electrolyte mixed with NH4F as the etching agent. The current density increased with voltage and the first steady state of the transient, that indicates the onset of chemical dissolution, was reached earlier when using higher quantities of NH4F. Changes in film pore size and morphology of un-etched regions of the NTAs correlated with the growth parameters. A flowchart that can be used to formulate set points, in order to detect deviations from desired anodization parameters, and hence declare a sample rejected or otherwise, is proposed for applications requiring a closed-loop control system to accurately monitor the anodic growth of TiO2 in ethylene glycol + NH4F electrolytes.

Designed roadblock for oxygen evolution through construction of stack structure during Ti anodization

Although many researches have involved oxygen generation to clarify the self-assembling growth of anodic TiO2 nanotubes, none convincing evidence has been provided to prove oxygen evolution during anodization. Herein, a special stack structure has been designed to restrain the digging behaviors of field-assisted dissolution theory. The stack structure looks like just a sandwich, made of lower porous layer, middle compact layer and upper porous layer, through anodizations for three times in different electrolytes. After the third anodization, the middle layer formed by the second anodization tends to an interlaced nanotube link layer, without any observation of one-to-one through-hole evidence from the cross-section FESEM images. The stack sandwich structure effectively rejects the digging manners of the traditional field-assisted dissolution theory, since the middle compact layer should be one-to-one through-hole after the fluorine dissolution process. Actually, the plastic oxide flow grows under the mould effects of oxygen bubbles and then results in the porous structure.

Flow-Regulated Growth of Titanium Dioxide (TiO2 ) Nanotubes in Microfluidics.

Electrochemical anodization of titanium (Ti) in a static, bulk condition is used widely to fabricate self-organized TiO2 nanotube arrays. Such bulk approaches, however, require extended anodization times to obtain long TiO2 nanotubes and produce only vertically aligned nanotubes. To date, it remains challenging to develop effective strategies to grow long TiO2 nanotubes in a short period of time, and to control the nanotube orientation. Here, it is shown that the anodic growth of TiO2 nanotubes is significantly enhanced (≈16-20 times faster) under flow conditions in microfluidics. Flow not only controls the diameter, length, and crystal orientations of TiO2 nanotubes, but also regulates the spatial distribution of nanotubes inside microfluidic devices. Strikingly, when a Ti thin film is deposited on silicon substrates and anodized in microfluidics, both vertically and horizontally aligned (relative to the bottom substrate) TiO2 nanotubes can be produced. The results demonstrate previously unidentified roles of flow in the regulation of growth of TiO2 nanotubes, and provide powerful approaches to effectively grow long, oriented TiO2 nanotubes, and construct hierarchical TiO2 nanotube arrays on silicon-based materials.

Electrode distance regulates the anodic growth of titanium dioxide (TiO2) nanotubes

Electrochemical anodization of titanium has been used widely to produce self-organized TiO2 nanotube arrays. Many experimental parameters, such as anodizing voltage and electrolyte composition, have been investigated extensively in the anodic growth of TiO2 nanotubes. The effect of electrode distance on the anodic growth of TiO2 nanotube arrays, however, remains elusive. This could be an important problem when in situ growth of TiO2 nanotubes is required in microdevices. Here, we show that decreasing the electrode distance at a constant anodizing voltage enhances the anodic growth of TiO2 nanotubes and the change of nanotube structures becomes more sensitive to the electrode distance at high voltages. We further demonstrate the correlation between electrode distance and current density during the anodic growth of TiO2 nanotubes and suggest that the change of current density regulated by electrode distance controls the growth of TiO2 nanotubes. The present study offers an effective approach to enhance the production of TiO2 nanotube arrays without changing the anodizing voltage and electrolyte composition and thus provides useful insights to the anodic growth of TiO2 nanotubes at reduced electrode distances.

Derivation of a Mathematical Model for the Growth of Anodic TiO2 Nanotubes under Constant Current Conditions

Due to certain limitations of traditional models, the growth mechanism of porous anodic TiO2 nanotubes has not been well determined currently. Herein, for the first time, a mathematical model of voltage-time transient curves under constant current conditions is derived theoretically, based on the conception of ionic and electronic currents and Ohm's law. The simulation results show high fidelity to the experimental curves, and illustrate the linear correlation between nanotube length and ionic current. Further, based on this model, the avalanche breakdown can be explained, which shows advantage over the former derivations on compact films. And this model indicates that the discrepancy between compact and porous oxide films lies on the magnitude of electronic current during anodization. Moreover, the proportion of the ionic and electronic current is then calculated during constant current anodization. It can be concluded that the ionic current contributes to the oxide growth while the electronic current gives rise to the oxygen bubble evolution which acts as the growth mould of the oxide. The present results promote the understanding of the growth kinetics of porous anodic oxides from qualitative interpretation to quantitative analyses.

A Mathematical Model for the Growth of Anodic TiO2 Nanotubes under Higher Current Density

The mechanism of titanium anodization is unclear yet. The total current includes ionic and electronic currents. The ionic current drives oxide film growth and the electronic current causes oxygen bubble emission. Based on a former derivation, a mathematical model of voltage-time transient curves during galvanostatic anodizing under higher current densities (>10 mA/cm2) was developed. The high-current model shares the same mechanism of oxide formation with the former low-current model, but the calculation of electric field is different. The results show high accuracy of fitted curves, and prove the linear correlation between nanotube length and ionic current. The two models vary in the applicable ranges, because the relation between ionic currents and electric field is different under high and low field.

Relationship between the Generation of Electronic Current and Film Morphology during Anodization of Ti

The formation mechanism of porous anodic oxides has been investigated for several decades. However, the current density-time curves for the growth of anodic TiO2 nanotubes (ATNTs) are still not well understood. It is known that the length of ATNTs increases with the anodizing current (Jtotal). However, shorter ATNTs are obtained at larger Jtotal while applying a high voltage, as Jtotal includes ionic current (Jion) and electronic current (Je). Though the Jtotal is larger at high voltages, its main part is Je in fact, which leads to oxygen evolution but makes no contribution to the growth of ATNTs. Further, compact oxides rather than nanotubes are obtained as the applied voltage decreases, which cannot be clarified by the field-assisted dissolution theory. Herein, we verify that Je generates at a critical thickness (dc) for the first time. ATNTs cannot form in fluoride-containing solutions at lower voltages, because the thickness of barrier layer cannot reach dc at lower voltages. The generation of Je at dc is the precondition to form nanotube embryos and the length of ATNTs is determined by the Jion instead of the Jtotal.

Atomic Layer Deposition and Anodic Oxidation: A Good Tool Combination to Build Nanostructured Electrodes for Energy Applications

Anodic oxidation has been used for a longtime as a way to passivate metallic electrodes to prevent corrosion. It is the discovery of self-ordered nanoporous alumina membranes reported in 1995 [1] that has revealed its potential use for surface nanostructuring. Later, the anodic growth of TiO2 nanotubes (TiO2-nt) has considerably widened the field of applications of such approach since TiO2 exhibits many valuable properties [2]. It is now possible to use other metals or alloys to grow porous or tubular oxidized nanostructures. However to further improve the properties of such electrodes, it is necessary to functionalize their surface with other materials of interest. Among the various thin film deposition methods, Electrochemical Deposition (ED) and Atomic Layer Deposition (ALD) have shown a great ability to conformally coat porous structures exhibiting a high aspect ratio. The main advantages of ALD over ED is that it can be carried out onto non-conductive materials, it allows an accurate control of the thickness and it is usually more direct to grow oxides or nitrides. We report, here, three examples of nanostructured electrodes fabricated using anodic oxidation of Al and Ti in combination with ALD of active materials. The targeted applications are in the fields of electrocatalysis, Li-ion microbatteries and photoelectrochemical water splitting. Direct Ethanol Fuel Cells offer significant advantages due to ethanol non-toxicity and renewability and its high power density. TiO2 have been successfully used as replacement of C as catalyst support because it exhibits a good chemical stability and it can enhance the activity of the catalyst. After a brief reminding on the TiO2-nt fabrication and properties, the ALD of Pd nanoparticles into TiO2-nt array will be presented. The electrochemical activity toward ethanol oxidation has been tested in alkaline medium. Although a high and stable electroactivity has been measured further improvements such as annealing of the TiO2-nt and ALD of SnO2 onto the TiO2-nt have been proposed. As seen on Fig. 1a, the electrochemical response is at its highest when the tubes have been annealed and covered by SnO2. A 3D nano-architectured composite negative electrode has been fabricated in order to be used in microbatterie. It consists of TiO2-nt coated by a thin SnO2 film grown by ALD. TiO2-nt are known to exhibits very good insertion properties and the SnO2 coating that displays a high lithium insertion capability (max. theoretical capacity of 720 µAh/cm2·µm) is used to enlarge the capacity of the electrode. Such composite 3D structure increases therefore the active area, improves the kinetics of the system, facilitates the ion exchange at the electrode/electrolyte interface and, better accommodates the volume expansion induced by the Li insertion within the LixSn alloy. The ALD of SnO2 will be described in details. The influence of various parameters such as precursor nature, exposure sequence as well as reaction temperature on the deposit morphology, chemistry and crystalline structure will be presented. The electrochemical performances of such systems have been tested as function of the SnO2 film thickness and after thermal treatments that change the crystalline structure of the electrodes. The results show that TiO2/SnO2 delivers a promising capacity (~150 µAh/cm2) which is better than bare TiO2-nt (Fig. 1b). It demonstrated then that such 3D nano-architectured composite electrode opens valuable perspectives for microbatteries design. The last example consists of producing a nanostructured photocathode for water splitting in which the photogeneration and transport of the charge carriers do not occur in the same material. In order to create tailored 3D nanostructures, p-NiO and i-Sb2S3 thin films have been successively grown into nanoporous alumina by ALD (Fig. 1c). The photocathode composition, structure and geometry are well controlled. Then the optical and photoelectrochemical properties have been investigated and optimized using cyclic voltammetry in the dark and under illumination as well as UV-visible absorption spectroscopy. It has been sought to uncover trends in the photoelectrochemical activity of the system (photocurrent density and photovoltage) and rationalize them in terms of optical and electrical functions. [1] H. Masuda, K. Fukuda, Science, 268, 1466 (1995) [2] V. Zwilling, E. Darque-Ceretti, A. Boutry-Forveille, D. David, M. Y. Perrin, M. Aucouturier, Surf. Interface Anal., 27, 629 (1999) Figure 1

Fast anodically growth of long, small diameter TiO2 nanotubes by electropolishing of Ti foils in an ethanol-containing solution

Anodic growth of TiO2 nanotubes with length of several 10µm in fluoride containing electrolytes conventionally leads to tube diameter of ca. 90–100nm with an undesired nanograss appearance on the surface of tubular array. Fabrication of nanotubes with diameters smaller than 50nm needs mild anodization conditions that lead to tubes shorter than 7µm. In the present work, we introduce a simple and fast approach for fabrication of highly ordered, long nanotubes, 23µm, with dramatically small diameter of ca. 29nm, free of nanograss and top porous layer just by 2h anodization. This method is based on the creation of small diameter semi-spheres on the Ti surface by electropolishing in an ethanol-containing electrolyte. Besides pretreatment of Ti foil, we show that controlled stirring speed and temperature of the electrolyte during anodization are crucial factors to achieve this novel morphology.

Growth of anodic TiO2 nanotubes in mixed electrolytes and novel method to extend nanotube diameter

• Correlation between compact anodic TiO 2 and porous anodic TiO 2 films is elucidated. • Ionic current is used to form oxide and increases the oxide forming efficiency. • Electronic current gives rise to O 2 gas and decreases the oxide forming efficiency. • A novel approach to enlarging the nanotube diameter is presented. • Nanotube diameters increase with increasing H 3 PO 4 amount in a given NH 4 F solution. It is well known anodic TiO 2 nanotubes (ATNTs) can be obtained by the anodization of Ti foils in fluoride-containing solutions, and the nanotube diameter is proportional to the applied voltages. However, the growth kinetics of ATNTs and the relationship between structural features and anodizing parameters still remain unclear. Challenges always remain in the fabrication of ATNTs with large diameters due to the undesired breakdown event under the high voltage in NH 4 F solutions. Here, an interesting approach is first proposed to overcome these particular challenges. A series of constant current anodizing processes in fluoride-free H 3 PO 4 solutions, NH 4 F solutions of different concentrations (0.7 wt%, 0.5 wt% and 0.2 wt%), and different mixed electrolytes containing both NH 4 F and H 3 PO 4 , have been compared in detail. And we mainly focused on the influence of the different ratios of NH 4 F and H 3 PO 4 on the outer diameters of ATNTs and the correlation between two types of films. The interesting results show that the nanotube diameter greatly increases with H 3 PO 4 amount in the solutions with a given concentration of NH 4 F. In contrast, the nanotube length decreases with the increase of H 3 PO 4 amount in the solutions with a given concentration of NH 4 F. The experimental findings and the undesired breakdown phenomenon can be elucidated by the theory of the electronic current and ionic current rather than by the field-assisted dissolution.

Controllable current oscillation and pore morphology evolution in the anodic growth ofTiO2 nanotubes

The spatial heterogeneities and temporal instabilities in the anodic growth ofTiO2 tubes are very important for nanostructure fabrication, but few orderedcases have been reported. In this work, we represent a new currentoscillation with pore morphology evolution in the formation of anodicTiO2 tubes. Small (less than 8.0% of the minimum value) and fast (period ∼ 100 s) current oscillation was formed under static conditions in a wide range,while significant morphological change such as periodical narrowing, abruptionand small pits appeared in the pore with characteristic length scale of101–102 nm. Surprisingly, the roughness in the pore would be totally eliminated instead of beingenhanced by high speed stirring or periodically modulated voltage with the currentoscillation still being enhanced, which indicates an important involvement of the iontransport process. It has also been found that the growth rate could be significantlyaccelerated by tuning the stirring rate or the periodical modulation of the voltage.The mechanism has been described with consideration of the local reactions andthe ion transport with a key involvement of the convection process, which canbe strongly influenced by the mechanical stirring and the modulated voltage.

Influence of Water Content on the Growth of Anodic TiO2 Nanotubes in Fluoride-Containing Ethylene Glycol Electrolytes

The effect of water additions, from , to an ammonium fluoride/ethylene glycol electrolyte on the composition and morphology of titania-based nanotubes was investigated by scanning and transmission electron microscopy, Rutherford backscattering spectroscopy, and nuclear reaction analysis. Further to the presence of units of , the films contained fluorine, carbon, nitrogen, and probably hydrogen species derived from the electrolyte. The compositions of the films appeared to be relatively independent of the water content of the electrolyte. Following a small increase in efficiency of film growth for additions between 0 and water, the efficiency decreased from about 25 to about 10% between water contents of 1 and , respectively, possibly due to a combination of loss of titanium species to the electrolyte and evolution of oxygen. The densities of the nanotube layers were estimated to be in the range of , with an average value of .