VFand tFdifferently influence the Ndvalues, showing that the defects formed in the TiO2during its growing
might be of different nature. When VFincreases, it causes an increment in the diffusion coefficient for the F
ions
inside the compact layer formed in the metal/oxide interface,
9
so a greater insertion of these ions is expected when
the TiO2nanotube films are grown at continuously higher VF. Modifying TiO2by doping with F
ions induce
energetic states inside the gap, provoking a displacement of the Efbtowards the conduction band of the TiO2,
increasing the Nd.
10,11
However, even when the same trend in Efb
is observed, the estimated Nddecreased as tFgot
increasingly higher, evidencing a further modification of the film probably related to the insertion of other species
present in the electrolyte, which is compensating the charge, inducing energeticstates in the gap over the valence