TiO2 + 6HF → H2[TiF6] + 2H2O

H2[TiF6] is soluble. HF concentration has effect on nanotube development rate.

HF acts as the capping and etching agents. That is to say, F- first selectively adsorb onto the specific facets of TiO2 to form a particular morphology, then the etching started with the substitution of surface -OH by F-, followed by -TiOF2 dissolution on the {001} facets into the aqueous solution with the formation of [TiF6]2-.

try with 10% HF(48%)