Multivariate Optimization of Self-doped TiO2 Nanotube Synthesis for Enhanced Optical and Electronic Performance

Abstract This paper presents the cathodic polarization of anodic TiO2 nanotube layers (TNT) on Ti to promote the electrochemical reduction of Ti4+ lattice into Ti3+, accompanied by oxygen-vacancy formation and proton intercalation, resulting in self-doped nanotubes (SD-TNT). Although this reduction route has been known since the early reports of TNT, the quantitative relationship between the polarization parameters and the resulting electronic response remains to be established. Hence, a full factorial design (FFD) coupled to response surface methodology (RSM) was employed to assess simultaneously the cathodic potential, the polarization (self-doping) time, and the calcination temperature, using the cathodic current density (jc) associated with the Ti4+/Ti3+ reduction as the response. The optimum condition, obtained by FFD/RSM, was −1.5 V, applied for 480 min, to layers calcined at 450 °C, which delivered a jc of −0.325 mA cm–2. Under these conditions, SD-TNT were also investigated using electrochemical impedance spectroscopy (EIS), which revealed a decrease of the charge-transfer resistance from 831.9 to 92.35 Ω, together with a lower capacitance, constant phase element exponent, and a shorter time constant, consistent with a higher density of Ti3+ centers and oxygen vacancies distributed along the tube axis. Raman deconvolution of the anatase Eg mode confirmed the increase of the oxygen-vacancy contribution after cathodic polarization, whereas the calcination temperature controlled the anatase/rutile ratio and, therefore, the apparent band gap. The multivariate approach, thus, provides quantitative criteria to select the cathodic polarization parameters of TNT.