The effect of the VF and tF over the photocurrent potentiostatically measured at 1.25 V vs Ag/AgCl in a 0.1 M HClO4 electrolyte is shown in Figure 2 b. In both cases, the photocurrent increase as VF and tF takes increasingly higher values, reaching a maximum for 30 V when the time length is fixed at 60 min (filled points) and at 120 min when the formation voltage is fixed at 30 V (filled squares). The formation of a maximum in the measured photocurrent with VF and tF does not correlate directly with the electronic properties of the film (Efb, Nd or Eg), see Table 2.
From the different morphological parameters reported in Table 1 the inner diameter of the nanotubes, di, is the only one that seems to show a monotonic dependence on the photoelectrochemical performance, exhibiting a maximum value around 71-77 nm, Figure 3. This dependence may be related to the fact that the diameter of the nanotubes determines the interaction of light with the film.15 Although various studies have linked the formation of the maximum to the morphology of the film, there is controversy about the optimal values of inner diameter and length of the nanotubes for photoelectrochemical applications,15 showing that these values can be related to other variables such as: the system used for photoelectrochemical characterization, the electrolyte used for anodizing, or to the subsequent heat treatment to which the film is subjected; that they are consequently altering the electronic properties of the films.