Electrochemical growth of titanium oxide nanotubes: the effect of surface roughness and applied potential

Aligned titanium dioxide nanotubes may be grown on the surface of titanium metal by electrochemical oxidation in the presence of fluoride ion. There are a number of salient parameters that have been reported to affect the nanotube growth i.e., the nature, pH and concentration of the fluoride electrolyte, the cell potential and process time for anodisation. Furthermore, it has been reported that the nanotubes as grown are amorphous and can be converted to a mixture of anatase and rutile crystalline phases by heat treatment at elevated temperatures. There have been no studies reported investigating the effect of surface roughness of the parent titanium metal on nanotube growth. In this work the electrochemical growth of titanium oxide nanotubes on titanium foil was investigated using an ammonium fluoride/ammonium sulphate electrolyte. The results confirm that the anodisation potential controls pore diameter. The surface coverage of nanotubes was dependent on the surface roughness of the parent titanium metal. AFM measurements on untreated titanium foil showed relatively high microscale roughness and low nanoscale roughness. SEM analysis of these samples showed nanotube growth to be confined to depressions or valleys on the surface and the nanotubes were of uniform pore diameter. Mechanically polishing the surface of the parent titanium decreased the microscale roughness and increased the nanoscale roughness which, resulted in more uniform surface coverage. However, this led to an increased variation in pore diameter and shape of the nanotubes. XRD was used to determine crystal structure before and after annealing at 460 degrees C.