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.

In vivo corrosion of four magnesium alloys and the associated bone response.

This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal-assisted chemical etching. First, the basic process and mechanism of metal-assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template-based metal-assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block-copolymer masks. The metal-assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal-assisted chemical etching is given, demonstrating the promising potential applications of metal-assisted chemical etching. Finally, some open questions in the understanding of metal-assisted chemical etching are compiled.

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Metal-Assisted Chemical Etching of Silicon: A Review

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 development of binary Mg–Ca alloys for use as biodegradable materials within bone

Self-ordering of the cell arrangement of the porous structure of anodic alumina has been studied in a sulfuric acid solution. Ordering of the cell arrangement was dependent on the applied potential, and a highly ordered structure was obtained under anodization at a constant potential of 25 to 27 V. Self-ordering of the porous structure proceeded with the growth of the oxide layer under anodization at an appropriate potential, and a porous film with an almost ideal hexagonal honeycomb structure was formed over an area of several micrometers after a long period of anodization.

Self‐Ordering of Cell Arrangement of Anodic Porous Alumina Formed in Sulfuric Acid Solution

Self-ordering of anodic porous alumina formed in organic acid electrolytes

The controlling factor of self-ordering of anodic porous alumina was investigated by focusing on the current density during film growth The homogeneity of cell size was improved with increasing formation voltage accompanied by the exponential increase in current density The maximum anodizing voltage for proceeding uniform oxide growth while avoiding extremely high current accompanied by gas evolution was identical with the previously established self-ordering voltage With the increase in formation voltage up to the self-ordering voltage, the ratio of pore diameter to cell diameter d pore /d cell lowered and converged to ∼03 regardless of the electrolyte type Moreover, domains of highly self-ordered pore arrays were found in the film formed during burning, where extremely high current was locally concentrated This suggests that the condition inducing film growth under high current density, ie, high electric field strength is the key controlling factor of self-ordering Based on the above knowledge, a new self-ordered porous alumina with a 600 nm pore interval was fabricated in citric acid just under the critical voltage of burning

https://iopscience.iop.org/article/10.1149/1.1767838/pdf

Controlling Factor of Self-Ordering of Anodic Porous Alumina

A TEM investigation of naturally formed oxide films on pure magnesium

Oxide films formed naturally on pure Mg are investigated by the use of ultramicrotomed cross sections and transmission electron microscopy. The film formed in air immediately after scratching the metal surface is initially thin, dense, amorphous, and relatively dehydrated. Continuing exposure to humid air or exposure to water leads to the formation of a thicker hydrated film adjacent to the metal. The film formed in water contains an additional top layer characterized by a plateletlike morphology. The structure of these layers and their significance in corrosion protection are discussed. The changes occurring in these structures as a result of exposure to the electron beam are reported.

https://iopscience.iop.org/article/10.1149/1.1837048/pdf

Sachiko Ono

Papers

Self‐Ordering of Cell Arrangement of Anodic Porous Alumina Formed in Sulfuric Acid Solution

Self-ordering of anodic porous alumina formed in organic acid electrolytes

Controlling Factor of Self-Ordering of Anodic Porous Alumina

A TEM investigation of naturally formed oxide films on pure magnesium

Morphology and Structure of Oxide Films Formed on Magnesium by Exposure to Air and Water