Titanium foil (0.1 mm thick) with a purity of 99.8% was used to
prepare the TNAs. After mechanically polished with abrasive paper
of different size, the titanium foil was rinsed in an ultrasonic bath
of acetone, ethanol, and deionized water in turn. The titanium foil
was chemically etched by immersing in a mixture of HF and HNO3
(HF:HNO3:H2O = 1:3:6 in volume, total 50 mL) for 10 s, followed by
rinsing in deionized water and finally drying in a N2 stream at room
temperature before anodization. The anodization was carried out
in a two-electrode cell with titanium foil as the working electrode
and platinum foil as the counter electrode at 20V under ambient
temperature. The electrolytes were 0.3 wt% NH4F and 2 vol% H2O in
ethylene glycol. After 2 h of anodization, the sample was immediatelyultrasonic
washed withdeionized water to remove the surface
debris and dried with a N2 stream. Then it was annealed at 773K
in air for 1 h to convert the amorphous phase to the crystalline one
for subsequent use.