We present comparatively the results obtained from two samples: one annealed under Nitrogen and the other in air. Photocurrent measurements and impedance spectroscopy reveal that the main mechanism responsible for the enhanced activity is the increased charge transfer rate at interface with the electrolyte, promoted by higher oxidation energy surface states. Interestingly, oxygen vacancies play a secondary role in this case, as demonstrated by the comparison of S1 and S2 samples obtained with very low Ti doping.