Surface self-doping capabilities in anodic Nb2O5 prepared by plasma electrolytic oxidation

Abstract

Self-doping is an effective strategy for introducing defects in metal oxides such as Nb2O5, thereby modifying their electrochemical properties. The formation of defects, such as oxygen vacancies, has been associated with decreased surface charge-transfer resistance and increased charge-carrier density. While this approach has been explored for TiO2 based materials, Nb2O5 remains less investigated in the literature. In this study, we investigate the effect of self-doping in Nb2O5 films obtained by plasma electrolytic oxidation (PEO) and its influence on later surface modification via silver deposition. The Nb2O5 films are produced by PEO using two different electrolyte solutions, oxalic acid and phosphoric acid, at parameters of 20 mA cm-2 and 15 ºC. The anodized samples are subsequently subjected to cathodic polarization (-1 V vs SHE for 150 s). Both pristine and self-doped samples are immersed in a silver nitrate solution at room temperature for 30 min. The samples are characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), Raman spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results reveal variations in morphology and composition, with samples anodized in phosphoric acid showing phosphorus incorporation from the electrolyte. Prior-reduced-anodized samples show different silver incorporation, at a significantly higher level in the sample anodized in oxalic acid (~ 18 at.%) than in the sample anodized in phosphoric acid (~ 4 at.%). Self-doping increases visible light absorption only for oxalic acid-based samples, as evidenced by the DRS spectra, suggesting defect formation. Raman analysis also shows increased structural disorder for oxalic acid-based samples after self-doping. Cyclic voltammetry measurements indicate a cathodic response for the sample anodized in oxalic acid, whereas the ones anodized in phosphoric acid exhibit predominantly capacitive behavior. EIS results show higher charge-transfer resistance for the samples containing phosphorus, likely due to phosphate species on the surface. Graphical abstract