This manuscript reports on the photoelectrochemical properties of anodized 0.1 and 0.15 at% Nb-doped NTs prepared using electrolytes containing different amount of water (2 to 12 wt%). It is shown that Nb-doped NTs perform better than undoped NTs, achieving a photocurrent density up to 1.35 mA/cm(2) at 1 V vs RHE when the water content is sufficiently high (12 wt%). The IPCE also confirms the superiority of Nb-doped NTs, again the larger values (55% IPCE at 300 nm) are obtained when using 12 wt% H2O in the electrolyte. EIS analysis and capacitance measurements highlight the peculiarity of the Nb-doped TiO2 NTs which behave differently than undoped NTs. The electron transport resistance in Nb-doped NTs is similar (10(3) Omega cm(2)) to the one of undoped NTs. However, Nb-doped NTs exhibit larger electron mobility and require lower activation energy for electron transport (i.e. a less negative bias) than undoped NTs. Furthermore, Nb-doping, combined with an optimal water content of 12 wt%, is found to be an effective strategy to fully suppress the formation of deep intra band-gap energy states. The presence of Nb in the TiO2 lattice dictates the fate and the energy of defects introduced during anodization and annealing. These defects are shallower than those observed in undoped NTs. (C) 2016 The Electrochemical Society. All rights reserved.