Niobium Oxide Nanorods Obtained by Hydrothermal Synthesis—Structure, Morphology, and Electrochemical Detection of Oxygen Via Oxygen Reduction Reaction

Abstract

Niobium oxides are promising materials for applications within various research fields, especially as electrocatalysts for various chemical reactions. The tuning of the synthetic parameters can achieve a successful compromise between morphology and structure, aiming to obtain certain properties. Hence, this study aimed to investigate the influence of hydrothermal synthesis parameters on the morphology and structure of niobium oxide growth on a niobium metallic plate. The effect of annealing on the material performance was also evaluated. Afterward, the most crystalline sample was tested for the electrochemical determination of dissolved oxygen, a fundamental reaction in corrosion, biomedicine, and environmental monitoring. This is the first work using this material configuration as an electrochemical sensor. The hydrothermal synthesis produced nanorods formed by poorly crystalline, acidic, hydrated Nb2O5. Increasing the mineralizer concentration could increase the crystallinity and the nanorod growth rate, but it could also promote a lack of structural and morphological uniformity throughout the surface. Heat treatment allowed the increase in crystallinity and favored orthorhombic Nb2O5. Raman spectroscopy revealed that, at the first moment, acidic, hydrated niobium oxide structures were formed as precursors of crystalline niobium oxide that would be developed with longer reaction times and a higher mineralizer concentration. The obtained niobium oxide showed electrocatalytic activity toward the oxygen reduction reaction, with comparable performance between the samples with and without heat treatment. At all analyzed pH values, the amperometric response was linearly correlated with the dissolved oxygen concentration. pH influenced the sensitivity of the material; a maximum sensitivity of 0.0417 mA/cm2·mg/L O2 was achieved at pH = 6. The participation of the acidic functionalities of the surface in the ORR reaction was confirmed by Raman spectroscopy.