Self-doping of Nb₂O₅NC by cathodic polarization for enhanced conductivity properties and photoelectrocatalytic...

A simple novel electrochemical reduction approach was developed for the self-doping of Nb⁴⁺ in niobium oxide nanochannels (Nb₂O₅NC), changing the conductivity, optical properties, and photocatalytic properties of the material. Nb₂O₅NC was synthesized using different electrolytes: 0.4 wt% HF in 1 M H₂SO₄ (EI), 0.4 M NH₄F in glycerol (EII), and 0.25 g NH₄F with 4 vol% water in glycol at 50 °C (EIII). Field emission scanning electron microscopy (FEG-SEM) analysis showed well-organized arrays of Nb₂O₅ nanochannels produced on Nb foil, with varying tube diameters in the order EII < EI < EIII and film thickness in the order EI < EII < EIII, which drastically affected the photocurrent vs. potential curves. In order to self-dope the Nb₂O₅, the samples were electrochemically reduced in 0.1 M KH₂PO₄ buffer solution (pH 10) for 5 min, at -2.5 V vs. Ag/AgCl, resulting in the doped samples denoted P-EI, P-EII, and P-EIII. The results showed that reduction of Nb⁵⁺ to Nb⁴⁺ occurred for all the Nb₂O₅NC samples, leading to decreased surface charge transfer resistance between the Nb₂O₅NC and the electrolyte, as well as increases of the charge carrier density and photocurrent for all the self-doped samples, compared to undoped samples. Sample P-EI was also tested for the degradation of reactive red 120 (RR120) dye, achieving efficient photoelectrocatalytic degradation of a 10 mg L^-1 dye solution. These results reveal that the self-doping approach can enhance the photoelectrocatalytic properties of Nb₂O₅ photoanode, offering an alternative way for the removal of reactive dyes.