The influence of three annealing atmospheres (air, nitrogen and argon) and the use of controlled hydrodynamic conditions (from 0 to 5000 rpm) on morphological, structural, chemical and photoelectrochemical properties of TiO2 nanotubes have been evaluated. For this purpose, different characterization techniques have been used: Raman Confocal Laser Spectroscopy, X-Ray Photoelectron Spectroscopy, Mott-Schottky analysis and photoelectrochemical water splitting tests. According to the results, it can be concluded that both hydrodynamic conditions and annealing in non-oxidizing atmospheres improve the photoelectrochemical response of the TiO2 nanotubes. This fact has been attributed to the oxygen vacancies formed after annealing in argon and nitrogen atmospheres and also to the presence of nitrogen into the TiO2 lattice due to the thermal treatment in the nitrogen atmosphere. Description of methods used for collection/generation of data: - Synthesis of TiO2 nanotubes by anodization Anodization of titanium was carried out at room temperature. Foranodization, the titanium sample was the anode and a platinum foilwas used as the cathode of the process. The anodization electrolytewas an ethylene glycol based solution with 0.05 M of NH4F and 1 Mof H2O. The titanium sample was connected to a Rotating Disk Electrode(RDE) to control the hydrodynamic conditions during anodization.Different rotation speeds were applied: 0 (stagnant), 2500 and5000 rpm. A multimeter in series was also connected to register theanodization current density during the process. A potential of 55 Vwas applied for 30 min. After anodization, the titanium sample wasrinsed with distilled water and ethanol and then dried with air. Finally,samples were cut in a slice of approximately 0.5 cm.In order to transform the amorphous TiO2 anodized nanostructures,a heat treatment was carried out. In this way, samples were annealedat 450 °C for 1 h to obtain anatase crystalline TiO2 nanostructures. Differentatmospheres were used (air, argon and nitrogen) to evaluate theinfluence of the atmosphere during the heat treatment. For annealingin argon and nitrogen atmospheres, samples were introduced in atubular oven and the gas was bubbled for 30 min prior the heat treatment,in order to remove the air contained in the tube. - Structural and compositional characterisation of the nanostructures: Raman Spectroscopy measurements were carried out with a ConfocalLaser microscope with Raman spectroscopy (WITec). In order todetermine the crystalline structure of the samples they were illuminatedwith a 633 nm (red laser) using 420 μW. X-Ray Photoelectron Spectroscopy spectra were collected using Al-K monochromatized radiation (1486.6 eV) at 3 mA×12 kV. The scanningstep energies were 200 eV to measure the whole energy band and50 eV to selectively measure elements. - Electrochemical characterisation of the nanostructures: Mott-Schottky plots under dark conditions (in the absence of illumination)for the samples anodized under hydrodynamic conditions(at 5000 rpm) and annealed in air, argon and nitrogen atmosphereswere obtained applying an initial potential of 0.5 VAg/AgCl and sweepingthe potential from that value to −0.4 VAg/AgCl at a frequency of5 kHz. The amplitude of the signal was 10 mV. Water splitting measurements were performed to evaluate which ofthe TiO2 nanostructures (obtained under different hydrodynamic conditionsand annealing atmospheres) presented the best photocurrentresponse. For this purpose, TiO2 nanostructures were used as photoanodesin the photoelectrochemical water splitting with sunlight. Photoelectrochemicalwater splitting tests were carried out in a threeelectrode electrochemical cell connected to a potentiostat, with a1 M KOH solution as electrolyte. The working electrode was theTiO2 nanostructure with an exposed area of 0.26 cm2, an Ag/AgCl(3 M KCl) was the reference electrode and a platinum foil was thecounter electrode. A potential scan from − 0.8 VAg/AgCl to 0.5 VAg/AgClwith a scan rate of 2 mV s−1 was performed and the photocurrent densityvalues generated were recorded by chopped light irradiation (60 sin the dark and 20 s in the light). Stability measurements were used to evaluate the resistance ofTiO2 nanostructures to photocorrosion. In these tests, a potential of0.5 VAg/AgCl was applied under light irradiation for one hour.