The variation of the band gap energy (Eg) with the processing variables is also summarized in Table 2. The increase in VF led to a shift towards increasing Eg values; while the increase in tF had the opposite effect, diminishing the estimated values for Eg. This behavior is in accordance with that observed in the variation of Nd in Table 2; because the insertion of C or N induces energetic states over the TiO2 valence band, decreasing Eg Additionally, according to previous studies, doping TiO2 with these elements leads to a shift of Efb towards more distant values from the TiO2 conduction band disagreeing with the trend observed in Table 2. However, it is worth mentioning that the measurement of semiconductor properties using EIS is limited to compact oxide film so in order to obtain information about the flat band potential of the nanotube film, open circuit potential measurements were performed under illumination (OCPon), see Figure 2a.
The increase in VF and tF during the growth of the TiO2 nanotubes modified the OCPon, showing that defects induced by the insertion of N and/or C during film growth, primarily impacts the properties of the porous outer film, and no the internal compact layer. This behavior is consistent with that observed by other researchers, that have shown than fluoride ions migrates faster than oxygen, or C and N species, due to their size and mobility in the TiO2 lattice; so fluoride ions are mainly present in the compact film, while species of C and N, remain TiO2 nanotubes