Self-Doped Nb2O5 Nanotube Arrays for Photoelectrochemical Water Treatment

Abstract

Amid the growing attention to the water pollution caused by recalcitrant organic micropollutants and concomitant demands on (industrial) wastewater reuse, photoelectrochemical (PEC) water treatment has been noticed as a feasible advanced oxidation process with minimal usage of chemicals. Inspired by previous research on blue TiO2 nanotube arrays (Blue-TNTs) with self-dopants (Ti3+) for an enhanced PEC activity, this study investigated an engineered Nb2O5 nanotube arrays (NNTs) for PEC degradation of aqueous organic pollutants. Pseudo-hexagonal NNTs were successfully prepared by anodization of Nb foils and subsequent annealing under N2 atmosphere, while an electrochemical self-doping (cathodization) modified the NNTs into an efficient photoanode with black coloration (to be noted as Black-NNTs). Compared with the intact NNTs, the Black-NNTs (inner diameter: ~39.41 nm; length: ~32.4 µm) showed similar morphology and X-ray diffraction pattern, whereas electron spin resonance (ESR) spectroscopy and X-ray photoelectron spectroscopy revealed ample oxygen vacancies in Black-NNTs. A series of (photo)electro-analysis indicated that the elevated donor density on Black-NNTs could improve the electrical conductivity. Accordingly, Black-NNTs could efficiently generate hydroxyl radicals, as confirmed by scavenging tests and ESR. The rate of methylene blue (MB) degradation on Black-NNTs in PEC (0.69 h-1) condition was greater than those in photocatalytic (PC, 0.30 h-1) and electrochemical (EC, 0.53 h-1) conditions. Moreover, the MB removal efficiency further increased (90%) by a current switching mode in order to overcome the chemical instability of the self-dopants under the anodic environment. On the other hand, band-edge shift and narrowed band gap (3.0 eV to 2.6 eV) brought about a photo-response of Black-NNTs towards visible light. Thus, under a visible light irradiation, total amount of MB degradation (under the current switching) on Black-NNTs was 39% higher than Blue-TNTs as a control. Consequently, relatively simple electrochemically self-doping was confirmed to be a viable way for the engineering of NNTs to be utilized for photoelectrochemical water treatment and reuse.