Next, the first anodization of the specimen was performed,
as shown in Fig. 1(c). The specimen was rinsed
several times in ethanol for more than 15 min, then
rinsed in deionized water, and finally dried in an air
stream. The anodization step was then carried out in
0.3 M oxalic acid solution. This anodization was performed
for more than 8 h. The experimental setup was
the same as that used in electro-polishing, except that
oxalic acid was used instead of the perchloric acid–ethanol
solution