Anodic oxide coatings with a hierarchical micronanostructure on sintered titanium powders

Abstract

TiO2 nanotubes formed by electrochemical anodising of Ti (titanium foil) are normally X-ray amorphous. To improve their functional properties, they are usually converted into crystalline nanotubes by annealing at T ≈ 400–500 °С. What is more, under certain conditions, oxide films with a hierarchical micronanostructure can be formed on titanium foil by anodising in fluorine-containing electrolytes. Such films contain nanostructured microcones whose atomic structure corresponds to anatase (a-TiO2). It is interesting to find out whether it is possible to form anodic oxide coatings with a hierarchical micronanostructure on the surface of sintered powders of titanium sponge, which should have much larger specific surfaces and a wider range of applications. This paper is aimed at the study of the process of anodising porous samples of sintered powders of titanium sponge in an aqueous electrolyte (1 М Н2SO4 + 0.15 wt % HF). The object of our study were sintered titanium powders in the form of samples of porous powder materials with a specific area of Ssp = 1,350 cm2/g. Anodising was conducted in a 1 М Н2SO4 + 0.15 wt % HF electrolyte at various values of current density (jm). Surface morphology before and after anodising was investigated by scanning electron microscopy and atomic force microscopy. X-ray diffractometry was used to study the phase composition. The research involved the study of the influence of conditions for the galvanostatic anodising of samples of porous powder materials made from titanium sponge on the growth, morphology, and atomic structure of anodic oxide coatings. For the first time, it was shown that anodising at the values of current density jm = (230÷1,890) mA/g leads to the appearance of nanostructured a-TiO2 microcones (with base diameters and heights of up to 4 μm) in an amorphous nanoporous/nanotube oxide matrix (with an effective pore/tube diameter of about 50 nm). Since such coatings have a high specific area and a hierarchical micronanostructure, they are promising for the design of devices for photocatalytic environment purification and production of superhydrophobic surfaces.

Keywords

• sintered powders
• porous powder materials
• titanium sponge
• anodic oxide coatings
• structure hierarchy
• microcones
• crystalline
• nanostructured titanium dioxide
• scanning electron microscopy
• atomic force microscopy