The effect of atmospheric pressure on the growth rate of TiO2 nanotubes: Evidence against the field-assisted dissolution theory

The growth mechanism of anodic TiO2 nanotubes has been a hot topic in recent years. The classical field-assisted dissolution (FAD) theory holds that the atmospheric pressure does not affect the growth of nanotubes, while the oxygen bubble model holds that the decrease of pressure is beneficial to the nanotube growth. In order to verify the oxygen bubble model, anodizing processes at three different pressures (0.1 MPa, 0.05 MPa and 0.005 MPa), three different currents and three different NH4F concentrations were studied. To the best our knowledge, some interesting results are obtained which cannot be obtained at standard atmospheric pressure (1 atm = 0.1 MPa). (1) In the same NH4F electrolyte, the same current density and the same temperature, the reduction of anodizing atmospheric pressure can improve the growth rate of TiO2 nanotubes. (2) Under the condition of the same temperature and current density, with the increase of NH4F concentration (0.3 wt%, 0.4 wt% and 0.5 wt%), the porosity of the nanotubes increases, and the growth rate of the nanotubes decreases obviously. These findings can serve as counterevidence to the FAD theory.