Review on the Formation of Anodic Metal Oxides and their Sensing Applications

Anodization of valve metal such as those of Al, Ti, and W among others have been extensively studied largely because of their unique morphology and extensive applications including gas and bio-sensing. While large volumes of published materials are available on the oxide of each metal, a concise review of previous works on these anodic oxides is timely. Herein, we present an overview of the formation process and applications (with emphasis to gas and bio-sensing) of anodic metal oxides that have been extensively researched. While porous and nonporous metal oxides have been produced and applied, the former has been given much attention as it provides more reactive surface area making it sine-qua-non in nanoscience and nanotechnology. The large effective surface area enables their applications as templates for the fabrication of periodic arrays of nanostructures, e.g., nanowires, nanodots, and nanotubes for various sensing technologies. Porous structures with different shape and size can be obtained by modulating the anodization conditions such as current, time, voltage, purity of metal, doping element, type and concentration of the electrolyte, electrolyte temperature and the pre-treatment of the metal substrate. The fabrication procedure, characterization and applications of each anodic metal oxide are presented in this review.