Among various material nanoarchitectures, the nanotube geometry has received incredible attention due to the unique properties provided by its high surface area as well as nanoscale wall thickness and the availability of a variety of techniques to fabricate them. Since the beginning of this century, anodization has emerged as one of the most effective techniques for the fabrication of functional oxide nanotubes. Oxide nanotubes of a number of metals and alloys have been developed using this versatile technique. We review here the research activities on anodic nanotubes of various binary, ternary, and multinary materials and their selected applications.

We report on the fabrication of self-organized titanium oxide nanotube arrays of enhanced surface area prepared by anodic oxidation of a pure titanium sheet in electrolyte solutions containing potassium fluoride (KF) or sodium fluoride (NaF). The effects of electrolyte composition and concentration, solution pH, and the anodic potential on the formation of nanotubes and dimensions of the resulting nanotubes are detailed. Although nanotube arrays of length greater than 500 nm are not possible with hydrofluoric acid containing electrolytes [G.K. Mor, O.K. Varghese, M. Paulose,N. Mukherjee, C.A. Grimes, J. Mater. Res. 18, 2588 (2003)], by adjusting the pH of a KF containing electrolyte to 4.5 using additives such as sulfuric acid, sodium hydroxide, sodium hydrogen sulfate, and/or citric acid, we could increase the length of the nanotube-array to approximately 4.4 μm, an order of magnitude increase in length. The as-prepared nanotubes are composed of amorphous titanium oxide. Independent of the electrolyte composition, crystallization of the nanotubes to anatase phase occurred at temperatures ⩾280 °C. Rutile formation occurred at the nanotube-Ti substrate interface at temperatures near 480 °C. It appears geometry constraints imposed by the nanotube walls inhibit anatase to rutile transformation. No disintegration of the nanotube array structure is observed at temperatures as high as 580 °C. The excellent structural and crystal phase stability of these nanotubes make them promising for both low- and high-temperature applications.

N-type nanocrystalline titania is a promising material for use in semiconductor photoelectrochemical cells and, potentially, the solar generation of hydrogen. In this study, we examined the photochemical properties of titania nanotube arrays made by anodization of a starting Ti foil in a fluoride ion containing electrolyte. The absorption properties of the titania nanotube samples were investigated using diffuse reflectance ultraviolet (UV)-visible (vis) spectroscopy, with a broadening of the absorption spectra seen as a function of material phase, nanotube diameter, and Pd sensitization. The magnitude of the anodic photocurrent obtained from the polycrystalline nanotube samples, measured under band gap UV illumination, appeared to be significantly higher than that reported for any other form of nanocrystalline titania. A maximum photoconversion efficiency (UV light exposure at 365 nm, intensity 146 mW/cm2) of 4.8% was obtained for 22 nm diameter nanotubes annealed at 500 °C and coated with a discontinuous palladium layer of 10 nm average effective thickness.

Titania nanotubes synthesized by a soft chemical process are described, having diameters of 8 nm to 10 nm and lengths ranging from approximately 0.1 μm to 1 μm. X-ray diffraction studies show the structure of the as-prepared nanotubes is the same as that of the starting anatase TiO2 nanoparticles. Energy-dispersive x-ray analysis and electron energy loss spectroscopy studies further indicate that the as-prepared nanotubes are composed of titania. Studies using transmission electron microscopy verified that the nanotubes are formed during alkali treatment, with subsequent acidic treatments having no effect on nanotube structure and shape.

A tremendous variation in electrical resistance, from the semiconductor to metallic range, has been observed in titania nanotube arrays at room temperature, ≈ 25°C, in the presence of low ppm hydrogen gas concentrations (≤ 1000 ppm). The nanotube arrays are fabricated by anodizing titanium foil in an aqueous fluoride containing electrolyte solution. Subsequently, the arrays are annealed in an oxygen ambient, then coated with a 10 nm layer of palladium by evaporation. Electrical contacts are made by sputtering a small (e.g. 1 mm diameter) platinum disk atop the Pd coated nanotube-array. These sensors exhibit a resistance variation of the order of over 107 (1,000,000,000%) in the presence of 1000 ppm hydrogen at 23°C. To the best of our knowledge this dynamic change in electrical resistance the largest known response of any material, to any gas, at any temperature. The sensors demonstrate complete reversibility, repeatability, high selectivity, no drift and wide dynamic range. The nanoscale geometry of the nanotubes, in particular the points of tube-to-tube contact, is believed to be responsible for the outstanding hydrogen gas sensitivities.

We report on non-particulate titania photoelectrodes with a unique highly-ordered nanotube-array architecture prepared by an anodization process that enables precise control over array dimensions. Under 320–400 nm illumination titania nanotube-array photoanodes, pore size 110 nm, wall thickness 20 nm, and 6 μm length, generate hydrogen by water photoelectrolysis at a normalized rate of 80 mL/W•hr, to date the most efficient titania-based photoelectrochemical device, with a conversion efficiency of 12.25%. The highly-ordered nanotubular architecture allows for superior charge separation and charge transport, with a calculated quantum efficiency of nearly 100% for incident photons with energies larger than the titania bandgap.

A tremendous variation in electrical resistance, from the semiconductor to metallic range, has been observed in titania nanotube arrays at room temperature, ≈25°C, in the presence of low ppm hydrogen gas concentrations (≤ 1000 ppm). The nanotube arrays are fabricated by anodizing titanium foil in an aqueous fluoride containing electrolyte solution. Subsequently, the arrays are annealed in an oxygen ambient, then coated with a 10 nm layer of palladium by evaporation. Electrical contacts are made by sputtering a small (e.g. 1 mm diameter) platinum disk atop the Pd coated nanotube-array. These sensors exhibit a resistance variation of the order of over 107 (1,000,000,000%) in the presence of 1000 ppm hydrogen at 23°C. To the best of our knowledge this dynamic change in electrical resistance the largest known response of any material, to any gas, at any temperature. The sensors demonstrate complete reversibility, repeatability, high selectivity, no drift and wide dynamic range. The nanoscale geometry of the nanotubes, in particular the points of tube-to-tube contact, is believed to be responsible for the outstanding hydrogen gas sensitivities.

In this study highly-ordered titania nanotube arrays of variable wall-thickness and length 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 composition and temperature. The nanotube length and wall thickness are key parameters influencing the magnitude of the photoanodic response and the overall efficiency of the water-splitting reaction. For 22 nm inner-pore diameter nanotube-arrays 6 μm in length, with 9 nm wall thickness, upon 320–400 nm illumination at an intensity of 100 mW/cm2, hydrogen gas was generated at the power-time normalized rate of 51 mL/hr•W at an overall conversion efficiency of 12.5%. To the best of our knowledge, this hydrogen generation rate is the highest reported for a titania-based photoelectrochemical cell.

Crystalline CuO and Cu2O nanowires with an average diameter of about 10 nm and lengths of several tens of microns were successfully synthesized, depending on synthesis conditions, using precursor Cu(OH)2 nanowires as templates. The crystallinity, purity, morphology, and structural features of the as-prepared nanowires were characterized by powder x-ray diffraction, selected-area electron diffraction, and high-resolution transmission electron microscopy. The results showed that the precursor polycrystalline Cu(OH)2 nanowires served as both reactants for the growth of CuO and Cu2O nanowires, and as templates controlling the size and shape of the resulting nanowires.

Using anodic oxidation with a time-dependent linearly varying anodization voltage, we have made films of tapered, conical-shaped titania nanotubes. The tapered, conical-shaped nanotubes were obtained by anodizing titanium foil in a 0.5% hydrofluoric acid electrolyte, with the anodization voltage linearly increased from 10-23 V at rates varying from 2.0-0.43 V/min. The linearly increasing anodization voltage results in a linearly increasing nanotube diameter, with the outcome being an array of conical-shaped nanotubes approximately 500 nm in length. Evidence provided by scanning electron-microscope images of the titanium substrate during the initial stages of the anodization process enabled us to propose a mechanism of nanotube formation.

Nanoporous tungsten oxide (WO3), with pores of 50 to 100 nm in diameter, has been obtained by galvanostatic (constant-current) anodization of tungsten in a 0.25 M oxalic acid electrolyte. At room temperature, the optimum current density for nanoporous formation is approximately 6.5 to 8 mA/cm2. Monitoring of the anodization voltage during the fabrication process reveals a close match with the theoretical model of Parkhutik et al. [V.P. Parkhutik and V.I. Shershulsky, J. Phys. D 25, 1258 (1992)] for growth of nanoporous Al2O3. The as-anodized films are amorphous and crystallize upon annealing at 350 °C in an oxygen atmosphere.

The stability of titanium oxide nanotube arrays at elevated temperatures was studied in dry oxygen as well as dry and humid argon environments. The tubes crystallized in the anatase phase at a temperature of about 280 °C irrespective of the ambient. Anatase crystallites formed inside the tube walls and transformed completely to rutile at about 620 °C in dry environments and 570 °C in humid argon. No discernible changes in the dimensions of the tubes were found when the heat treatment was performed in oxygen. However, variations of 10% and 20% in average inner diameter and wall thickness, respectively, were observed when annealing in a dry argon atmosphere at 580 °C for 3 h. Pore shrinkage was even more pronounced in humid argon environments. In all cases the nanotube architecture was found to be stable up to approximately 580 °C, above which oxidation and grain growth in the titanium support disrupted the overlying nanotube array.

Sol-gel-derived metal oxide ceramic thin films deposited onto amorphous iron-rich substrates were found to form self-organized nanoporous structures dependent upon the extent to which the substrate is de-alloyed, a function of the substrate alloycomposition, acid concentration of the sol, and film drying conditions. Field emission scanning electron microscopy, transmission electron microscopy, and x-ray energy dispersive analysis were used to investigate details of the porous structure formation.Our studies showed the more electrochemically active elements in the amorphous substrate are de-alloyed by the sol in high-humidity environments, whereupon the liberated elements form oxides replicating the de-alloyed substrate matrix resulting in athree dimensional porous network structure.

Titanium oxide nanotubes were fabricated by anodic oxidation of a pure titanium sheet in an aqueous solution containing 0.5 to 3.5 wt% hydrofluoric acid. These tubes are well aligned and organized into high-density uniform arrays. While the tops of the tubes are open, the bottoms of the tubes are closed, forming a barrier layer structure similar to that of porous alumina. The average tube diameter, ranging in size from 25 to 65 nm, was found to increase with increasing anodizing voltage, while the length of the tube was found independent of anodization time. A possible growth mechanism is presented.

A magnetically-driven method for controlling nanodimensional porosity in sol-gel-derived metal–oxide films, including TiO2, Al2O3, and SnO2, coated onto ferromagnetic amorphous substrates, such as the magnetically-soft Metglas1 alloys, is described. On the basis of the porous structures observed dependence on external magnetic field, a model is suggested to explain the phenomena. Under well-defined conditions it appears that the sol particles coming out of solution, and undergoing Brownian motion, follow the magnetic field lines oriented perpendicularly to the substrate surface associated with the magnetic domain walls of the substrate; hence the porosity developed during solvent evaporation correlates with the magnetic domain size.