Six samples of titania nanotubes with different sodium content
were prepared by a hydrothermal alkali treatment of a titania precursor
and subsequent washing with HCl aqueous solutions at different
concentrations. As determined by EDX analysis, the sodium
content in the TNT(x) samples varied from 7.1 to 0.1 wt.% (Table 1).
From this result, it can be concluded that the sodium ions were not
replaced completely by protons after the nanotubes were washed
with the HCl aqueous solution and deionized water. As expected,
an increase in the concentration of HCl used in the acid treatment
resulted in TNT samples with smaller amounts of residual Na+
.
Thus, after one washing of the starting sodium-containing nanotubes
with a 0.001 M HCl solution, the residual sodium content
decreased to 7.1 wt.%, whereas the use of a 0.1 M acid solution
resulted in TNT materials with 0.7 wt.% of Na. To obtain sodium
content lower than 0.7 wt.%, the starting TNT sample was washed
twice with a 0.1 M HCl solution, giving nanotubular material with
only 0.1 wt.% of Na. Assuming the existence of sodium in the nanotubes
and a significant amount of water, the synthesized materials
can be described as trititanates of the general formula (H,
Na)2Ti3O7xH2O