Actually, when using ammonium fluoride as F ions source for obtaining nanotubes you don't actually have a N-doping effect. Usually, you just have some nitrogen inside (left from the electrolyte). For more information check the following review by P. Roy et al.: TiO2 nanotubes: Synthesis and applications, from 2011 in Angew. Chemie Int. Ed.

Just have in mind that best methods to dope nanotubular structures of TiO2 or other metals, are by thermal treatments using a gas able to deliver N to the substrate (e.g. ammonia). Proof that there is N-doping and that it is active is XPS and most important is the peak at about 396 eV, which shows that the N is interstitial. You can actually put other N containg compounds in the electrolyte, but just by doing that you will have more N but not ensuring a doping effect.