Atomic layer deposition: an overview.

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

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.

/pdf/titanium-oxide-nanotube-arrays-prepared-by-anodic-oxidation-29aqfhzd78.pdf

Titanium oxide nanotube arrays prepared by anodic oxidation

A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications

Self-organized hexagonal pore arrays with a 50–420 nm interpore distance in anodic alumina have been obtained by anodizing aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays with distances as large as 420 nm were obtained under a constant anodic potential in phosphoric acid. By comparison of the ordered pore formation in the three types of electrolyte, it was found that the ordered pore arrays show a polycrystalline structure of a few micrometers in size. The interpore distance increases linearly with anodic potential, and the relationship obtained from disordered porous anodic alumina also fits for periodic pore arrangements. The best ordered periodic arrangements are observed when the volume expansion of the aluminum during oxidation is about 1.4 which is independent of the electrolyte. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighboring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.

http://www-old.mpi-halle.mpg.de/mpi/publi/pdf/2438_98.pdf

Hexagonal pore arrays with a 50-420 nm interpore distance formed by self-organization in anodic alumina

The conditions for the self-organized formation of ordered hexagonal structures in anodic alumina were investigated for both oxalic and sulfuric acid as an electrolyte. Highly ordered pore arrays were obtained for oxidation in both acids. The size of the ordered domains depends strongly on the anodizing voltage. This effect is correlated with a voltage dependence of the volume expansion of the aluminum during oxidation and the current efficiency for oxide formation. The resulting mechanical stress at the metal/oxide interface is proposed to cause repulsive forces between the neighboring pores which promote the formation of ordered hexagonal pore arrays.

/pdf/self-organized-formation-of-hexagonal-pore-arrays-in-anodic-izy2ovbidd.pdf

Self-organized formation of hexagonal pore arrays in anodic alumina

The morphology and formation conditions of ordered hexagonal pore arrays in anodic alumina are discussed. The ordered arrangement of pores is shown to form by a self-organized process starting from randomly distributed pore positions at the surface of the alumina. The influence of the pretreatment of the aluminum substrate and the anodizing conditions on the growth kinetics and the tendency to form hexagonal pore structures were investigated. Homogeneous etching conditions are required in order to obtain regular pore arrays. This observation corresponds to the finding that hexagonal pore arrays are related to a smooth etching front and a homogeneous depth of neighboring pores.

/pdf/self-organized-formation-of-hexagonal-pore-structures-in-3ytd2alrau.pdf

O. Jessensky

Papers

Self-organized formation of hexagonal pore arrays in anodic alumina

Self-Organized Formation of Hexagonal Pore Structures in Anodic Alumina

Microstructure and photoluminescence of electrochemically etched porous SiC