Nitrogen doped TiO2 nanotube arrays were prepared by anodizing Ti foils in an organic electrolyte containing specified amounts of urea as nitrogen precursor. The photocatalytic activity of the samples was evaluated by analyzing the degradation kinetics of phenol in water. The influence of tubes' length, tubes' surface morphology and amount of nitrogen in the TiO2 lattice on hydroxyl radical formation efficiency, photocatalytic activity and stability in four cycles was investigated. It was found that the photocatalytic activity as well as the charge carrier recombination rate depends on nitrogen concentration and the process parameters. 3.5 mu m-long nanotubes containing 0.34 at.% of nitrogen seems to be favorable in phenol degradation and OH radicals generation under visible light. Comparison of XPS and photocatalytic activity test results shows decrease in phenol degradation efficiency with increasing amount of carbon contaminants on photocatalysts' surface. (C) 2016 Elsevier B.V. All rights reserved.