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.     

Nanoporous anodic alumina obtained without protective oxide layer by hard anodization

In this study, a fast and cost-effective approach is applied for fabricating nanoporous anodic alumina membranes under hard conditions without a protective oxide layer. This structural characteristic is a result of using a two-step anodization strategy under specific hard conditions during the second anodization step (i.e. high stirring rate, low acid electrolyte temperature and concentration). Notice that, after the anodization process, the membrane is detached from the aluminium substrate at the same time that pores are opened. So, after the fabrication process, no additional stages are required for removing both the protective oxide layer and the oxide barrier layer from the top and the bottom of the membrane, respectively. The resulting nanostructures obtained by this approach are defect-free nanoporous anodic alumina membranes with well-defined pores from the top to the bottom. This makes it possible to directly use those membranes in later applications (e.g. templates for replicating nanostructures, filters, optoelectronic devices and so forth) without additional processes and costs.     

Electrochemical corrosion behaviors of titanium covered by various TiO<Subscript>2</Subscript> nanotube films in artificial saliva

TiO₂ nanotube films obtained by anodization have shown great promise as biomaterials. In the present work, we report on the corrosion behaviors of titanium (Ti) with various TiO₂ nanotubes prepared by using controlled anodization procedures. Special emphasis is put on the impact of film morphologies on the corrosion resistance of the Ti substrate. The corrosion behaviors of Ti with different nanotube films were studied in artificial saliva using open-circuit potential measurement, potentiodynamic polarization, and electrochemical impedance spectroscopy techniques. Ti covered by TiO₂ nanotube films showed the markedly enhanced corrosion resistance properties compared to bare Ti. The existence of the compact oxide layer formed in a fluoride-free electrolyte was found to be beneficial for improving corrosion resistance properties. Besides, the TiO₂ nanotube films obtained by two-step anodization had better corrosion resistance than those obtained by single-step anodization, though they used the identical anodization parameters.     

电解液性质对TiO2纳米管阵列形貌及生长机理的影响

采用电化学阳极氧化法在纯钛片表面制备出了结构整齐有序的TiO₂纳米管阵列, 主要研究了电解液的性质、浓度以及氧化时间对TiO₂纳米管阵列形貌的影响, 并对不同电解液中TiO₂纳米管阵列的形成机理进行了初步探讨. 结果表明：在不同浓度的HF酸电解液中均可制备出规则、均匀的TiO₂纳米管阵列, 管径均匀, 表面平整, 但是纳米管的长度均较短, 约为300～350nm. 在高浓度HF电解液中, 同时获得了规则的纳米管阵列和纳米棒阵列. 在0.5wt% NaF和1mol/L Na₂SO₄中性电解液中也可以制备出表面光洁、排列整齐有序的TiO₂纳米管阵列, 纳米管长度明显长于HF酸电解液中获得的纳米管阵列, 达到了700nm, 但是阵列的表面平整度较差. 在乙二醇+0.6wt% NH₄F+2vol% H₂O有机电解液体系中可以制得超长的TiO₂纳米管阵列, 管径在150nm左右, 管长可达6μm.     

Fabrication of Gradient TiO2 Nanotubes on Ti Foil by Anodization

A new anodization based method is developed to fabricate a gradient in TiO₂ nanotube diameters and lengths on Ti foil. In the method, the applied anodization voltage is increased step by step, while the Ti foil is immersed into an aqueous solution of hydrofluoric acid progressively during anodization. The gradient TiO₂ nanotubes with tube diameters ranging from 55 to 105 nm and lengths ranging from 300 to 500 nm across 12 mm of Ti foil are obtained. The formation mechanism of gradient diameter and length is also discussed. The gradient structure is potentially useful as new cell instructive materials (CIMs) for guided cell movement and culture, novel drug delivery vehicles, size‐selective biosensors, or other sensor applications.     

Investigations on the self-organized growth of TiO2 nanotube arrays by anodic oxidization

The formation mechanism of a thin film of self organized TiO₂ nanotube arrays prepared by anodic oxidization of a pure titanium sheet in electrolyte solutions containing potassium fluoride and sulfate was investigated through near-real time monitoring the anode mass, the current density, and the surface topography during the anodization. Energy dispersive X-ray spectrometry was used to monitor the surface composition change during the anodization. The titanium surface was first electrochemically oxidized to form a layer of dense oxide under which nanotubes were originated. With the protection of the oxide layer, long nanotubes could be formed in electrolyte solutions with relatively high pH. The surface composition analysis indicates that the nanotubes were not totally oxidized to TiO₂. However, no other elements but Ti and O were found in the oxide film. This work reveals a way to fabricate long nanotubes with defined sizes.     

Influence of secondary anodization voltage on free-standing crystallized TiO2 nanotube array membrane

Vertically aligned, free-standing crystallized TiO₂ nanotube arrays with a length of 32 μm have been fabricated by a two-step anodization method. The TiO₂ nanotube membrane can be detached from the Ti substrate through the secondary anodization process. The influence of the secondary anodization voltage on the morphology, crystalline phase and photovoltaic performance of the as-fabricated samples has been investigated. Results show that the side wall of TiO₂ nanotubes becomes obviously thin as the secondary anodization voltage increases and leads to crack when the voltage reaches 25 V. The mass fraction of the anatase reduces by the increase of the voltage. Furthermore, the dye-sensitized solar cells (DSSCs) based on TiO₂ nanotube arrays have been assembled. The energy conversion efficiency decreases with the increase of secondary anodization voltage, and a highest energy conversion efficiency of 10.6 % under UV illumination (368.1 nm) is obtained from the cell with TiO₂ nanotube membrane re-anodizad at 15 V.     

Self-organized highly ordered TiO2 nanotubes in organic aqueous system

A simple method to achieve self-organized, freestanding TiO₂ nanotube array was constructed, free of corrosive etching process which was traditionally employed to separate TiO₂ nanotubes from the metallic Ti substrate. The TiO₂ nanotube arrays were constructed through potentiostatic anodization of Ti foil in aqueous electrolyte containing NH₄F and ethylene glycol. The nanotubes in the array were of 45 μm lengths and 100 nm average pore diameters. The effect of NH₄F concentration on the length of the self-organized nanotube arrays was investigated. Electrochemical and spectroscopic measurements showed that the as-prepared nanotubes possessed large surface areas, good uniformity, and were ready for enzyme immobilization. The as-prepared nanotube arrays were amorphous, but crystallized with annealing at elevated temperatures, as demonstrated by X-ray diffraction (XRD).     

TiO2纳米环/纳米管分层结构的生长机理及其吸光特性

由于电解液成分、氧化条件和所制备纳米管尺寸等因素的影响, TiO₂纳米管阵列在阳极氧化过程中容易形成纳米草结构, 而利用两步阳极氧化法可制备出表面整洁的TiO₂纳米环/纳米管分层结构。实验通过控制氧化时间得到了不同生长阶段的TiO₂纳米环/纳米管分层结构, 并对其生长机理及吸光特性展开研究。结果表明, 在第二步阳极氧化过程中, 钛箔表面的周期性六边形纳米洞结构可以使纳米管的生长过程局限于每个纳米洞内部, 从而形成TiO₂纳米环/纳米管分层结构。同时, 存在的纳米环可以为其内部的纳米管提供支撑作用, 防止纳米草结构的形成。TiO₂纳米环/纳米管分层结构的吸收光谱在可见光区域呈现震荡形态, 这是纳米环顶部反射的光与钛基底反射的光相互干涉导致的。根据震荡峰形状与样品厚度的关系, 可以估算出入射光在TiO₂纳米环/纳米管分层结构中的最大穿透深度为2 μm左右。     

阳极氧化法制备有序纳米多孔氧化铝膜的研究

利用电化学阳极氧化的方法,在草酸溶液中,精确控制反应条件,在高纯铝片表面有序生长了纳米多孔氧化铝膜。试验中,分别采用一次阳极氧化和二次阳极氧化方法制备氧化铝膜。利用H3PO4溶液浸泡法对氧化铝膜进行扩孔处理。通过扫描电子显微镜对样品进行表征分析。结果发现,二次阳极氧化制备的氧化铝膜的孔洞分布较一次氧化的更为规则有序,并且孔径大小均匀一致。扫描电镜观察显示,氧化铝膜的扩孔过程可以去掉阻碍层,并调节孔径大小,溶去二次氧化后黏附在氧化层表面的一些杂质,从而使氧化铝模板更为规则有序,孔径均一。这种经过二次阳极氧化和扩孔处理得到多孔阳极氧化铝模板的方法简单,成本较低,可以为后续的纳米材料合成提供高质量的合成模板。     

Study of nanopores ordering in anodic aluminum oxide templates achieved by three step process

Enhancement of naturally-occurred self ordering nanopores in anodic aluminum oxide membrane by performing three-step anodic oxidation process has been reported. Naturally-occurred self ordering of nanopores in anodic aluminum oxide membrane has brought it into the applications of template for fabrication of nanoscale materials. Three-step anodic oxidation method was used to achieve self-ordering of nanopores. Effect of duration of first and second steps on the ordering of nanopores was investigated. The current-time curves recorded during anodization elucidate an almost same behavior for all three steps. Scanning electron micrographs show hexagonally arranged 45 nm pores in a manner which contribute into the formation of highly ordered areas, called domains. Larger ones are clearly observed over the surface, for samples with longer first and second anodization steps.     

In situ growth of single-crystal TiO2 nanorod arrays on Ti substrate: Controllable synthesis and photoelectrochemical water splitting

Despite one-dimensional (1D) semiconductor nanostructure arrays attracting increasing attention due to their many advantages,highly ordered TiO2 nanorod arrays (TiO2 NR) are rarely grown in situ on Ti substrates.Herein,a feasible method to fabricate TiO2 NRs on Ti substrates by using a through-mask anodization process is reported.Self-ordered anodic aluminum oxide (AAO) overlaid on Ti substrate was used as a nanotemplate to induce the growth of TiO2 NRs.The NR length and diameter could be controlled by adjusting anodization parameters such as electrochemical anodization voltage,anodization time and temperature,and electrolyte composition.Furthermore,according to the proposed NR formation mechanism,the anodized Ti ions migrate and deposit in the AAO nanochannels to form Ti(OH)4 or amorphous TiO2 NRs under electric field,owing to the confinement effect of the template.Photoelectrochemical tests indicated that,after hydrogenation,the TiO2 NRs presented higher photocurrent density under simulated sunlight and visible light illuminations,suggesting their potential use in photoelectrochemical water splitting,photocatalysis,solar cells,and sensors.     

Fast fabrication of long TiO2 nanotube array with high photoelectrochemical property on flexible stainless steel

Oriented highly ordered long TiO2 nanotube array films with nanopore structure and high photoelectrochemical property were fabricated on flexible stainless steel substrate (50 microm) by anodization treatment of titanium thin films in a short time. The samples were characterized by means of field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and photoelectrochemical methods, respectively. The results showed that Ti films deposited at the condition of 0.7 Pa Ar pressure and 96 W sputtering power at room temperature was uniform and dense with good homogeneity and high crystallinity. The voltage and the anodization time both played significant roles in the formation of TiO2 nanopore-nanotube array film. The optimal voltage was 60 V and the anodization time is less than 30 min by anodizing Ti films in ethylene glycerol containing 0.5% (w) NH4F and 3% (w) H2O. The growth rate of TiO2 nanotube array was as high as 340 nm/min. Moreover, the photocurrent-potential curves, photocurrent response curves and electrochemical impedance spectra results indicated that the TiO2 nanotube array film with the nanoporous structure exhibited a better photo-response ability and photoelectrochemical performance than the ordinary TiO2 nanotube array film. The reason is that the nanoporous structure on the surface of the nanotube array can separate the photo electron-hole pairs more efficiently and completely than the tubular structure.     

Amorphous and crystalline TiO2 nanotube arrays for enhanced Li-ion intercalation properties

We have employed a simple process of anodizing Ti foils to prepare 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 480 degrees C. Rutile crystallites emerge in the anatase nanotube when the annealing temperature is increased to 580 degrees C, resulting in TiO2 nanotubes of mixed phases. The morphological features of nanotubes remain unchanged after annealing. Li-ion insertion performance has been studied for amorphous and crystalline TiO2 nanotube arrays. Amorphous nanotubes with a length of 3.0 microm and an outer diameter of 125 nm deliver a capacity of 91.2 microA h cm(-2) at a current density of 400 microA cm(-2), while those with a length of 25 microm and an outer diameter of 158 nm display a capacity of 533 microA h cm-2. When the 3-microm long nanotubes become crystalline, they deliver lower capacities: the anatase nanotubes and nanotubes of mixed phases show capacities of 53.8 microA h cm-2 and 63.1 microA h cm(-2), respectively at the same current density. The amorphous nanotubes show excellent capacity retention ability over 50 cycles. The cycled nanotubes show little change in morphology compared to the nanotubes before electrochemical cycling. All the TiO2 nanotubes demonstrate higher capacities than amorphous TiO2 compact layer reported in literature. The amorphous TiO2 nanotubes with a length of 1.9 microm 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. These results suggest that anodic TiO2 nanotube arrays are promising electrode materials for rechargeable Li-ion batteries.     

Enhanced photocleavage of water using titania nanotube arrays

In this study highly ordered titania nanotube arrays of variable wall thickness are used to photocleave water under ultraviolet irradiation. We demonstrate that the wall thickness and length of the nanotubes can be controlled via anodization bath temperature. We find that the nanotube wall thickness is a key parameter influencing the magnitude of the photoanodic response and the overall efficiency of the water-splitting reaction. For 22 nm inner pore diameter nanotube arrays, those fabricated in a 5 degrees C anodization bath, 224 nm length and 34 nm wall thickness produced a photoanodic response that was thrice that of a nanotube array fabricated in a 50 degrees C anodization bath, 120 nm length and 9 nm wall-thickness. At high anodic polarization, above 1 V, the quantum efficiency under 337 nm illumination was greater than 90%. For the 5 degrees C anodization bath samples (22 nm pore-diameter, 34 nm wall thickness), upon 320-400 nm illumination at an intensity of 100 mW/cm(2), hydrogen gas was generated at the power-time normalized rate of 960 micromol/h W (24 mL/h W) at an overall conversion efficiency of 6.8%. To the best of our knowledge, this hydrogen generation rate is the highest reported for a titania-based photoelectrochemical cell.     

End-closed NiCoFe-B nanotube arrays by electroless method

A novel approach is obtained during the fabrication of NiCoFe-B nanotube arrays via electroless method. Porous anodic aluminum oxide (AAO) templates fabricated by anodization of aluminum foil were sensitized using PdCl₂ solution and immersed into electroless plating baths at room temperature to produce nanotube arrays. Compositional and morphological properties of the nanotube arrays are characterized. Results indicates the formation of end-closed nanotubes with the dimension of 100-130 nm in outside diameter, which is determined by the pore size of the AAO template, and about 15 nm in thickness of tube walls. The possible formation mechanism of end-closed metallic nanotube arrays is discussed.     

Photochemical performance and electrochemical capacitance of titania nanocomplexes

Titania nanocomplexes, comprising the disordered nanoribbons or nanowires on the top surface and highly ordered nanotube array on the underlaying layer, has been fabricated by longitudinally splitting off nanotubes in a controlled anodization process. Anatase titania nanocomplexes show higher photovoltage and photocurrent responses and photocatalysis activity than titania nanotube array due to the enhanced light harvesting caused by nanoribbons and nanowires. Furthermore, titania nanowire-nanotube demonstrates a higher photoelectrical performance than nanoribbon-nanotube due to its thicker space charge layer caused by long nanotubes and more effective surface area contributed by nanowires. Cyclic charge-discharge measurements show that titania nanotube array exhibits a much higher electric double layer capacitance than titania nanocomplexes because the surface nanoribbons or nanowires inhibit the free diffusion and transportation of electrolyte ions into the underlaying nanotubes. Therefore, titania nanocomplexes can act as a photoactive material for photocatalysis applications and titania nanotube array can act as an electrode substrate for electrochemical supercapacitor applications.     

Mechanical constraint and release generates long, ordered horizontal pores in anodic alumina templates

We describe the formation of long, highly ordered arrays of planar oriented anodic aluminum oxide (AAO) pores during plane parallel anodization of thin aluminum 'finger' microstructures fabricated on thermally oxidized silicon substrates and capped with a silicon oxide layer. The pore morphology was found to be strongly influenced by mechanical constraint imposed by the oxide layers surrounding the Al fingers. Tractions induced by the SiO(2) substrate and capping layer led to frustrated volume expansion and restricted oxide flow along the interface, with extrusion of oxide into the primary pore volume, leading to the formation of dendritic pore structures and meandering pore growth. However, partial relief of the constraint by a delaminating interfacial fracture, with its tip closely following the anodization front, led to pore growth that was highly ordered with regular, hexagonally packed arrays of straight horizontal pores up to 3 μm long. Detailed characterization of both straight and dendritic planar pores over a range of formation conditions using advanced microscopy techniques is reported, including volume reconstruction, enabling high quality 3D visualization of pore formation.     

Effect of TiO2 nanotube length and lateral tubular spacing on photovoltaic properties of back illuminated dye sensitized solar cell

The main objective of this study is to show the effect of TiO₂ nanotube length, diameter and intertubular lateral spacings on the performance of back illuminated dye sensitized solar cells (DSSCs). The present study shows that processing short TiO₂ nanotubes with good lateral spacings could significantly improve the performance of back illuminated DSSCs. Vertically aligned, uniform sized diameter TiO₂ nanotube arrays of different tube lengths have been fabricated on Ti plates by a controlled anodization technique at different times of 24, 36, 48 and 72?h using ethylene glycol and ammonium fluoride as an electrolyte medium. Scanning electron microscopy (SEM) showed formation of nanotube arrays spread uniformly over a large area. X-ray diffraction (XRD) of TiO₂ nanotube layer revealed the presence of crystalline anatase phases. By employing the TiO₂ nanotube array anodized at 24?h showing a diameter ??80?nm and length ??1·5???m as the photo-anode for back illuminated DSSCs, a full-sun conversion efficiency (??) of 3·5 % was achieved, the highest value reported for this length of nanotubes.     

Hydrothermal growth of free standing TiO2 nanowire membranes for photocatalytic degradation of pharmaceuticals

Highly entangled TiO(2) nanowires were directly synthesized by hydrothermal growth on Ti substrates at 180 °C utilizing various organic solvents to oxidize Ti. The growth mechanism, microstructure and phase transition of TiO(2) nanowire membranes were investigated in detail. TiO(2) nanowires, with diameters of 10-20 nm and lengths up to 100 μm, show a phase transition from Type-B to anatase by annealing at 700 °C. Robust, free standing TiO(2) nanowire membranes with millimeter level thickness can be cleaved from Ti substrates or directly prepared from thin Ti foils. These porous TiO(2) membranes, while effective for mechanical microfiltration, can also photocatalytically degrade pharmaceuticals such as trimethoprim under UV irradiation.