Enhanced Photocatalytic Performance of Nitrogen-Doped TiO₂ Nanotube Arrays Using a Simple Annealing Process

• Main Authors: Phuoc Huu Le, Le Trung Hieu, Tu-Ngoc Lam, Nguyen Thi Nhat Hang, Nguyen Van Truong, Le Thi Cam Tuyen, Pham Thanh Phong, Jihperng Leu
• Format: Article
• Language: English
• Published: MDPI AG 2018-11-01
• Series: Micromachines
• Subjects: N-doped TNAs; two-step anodization; photocatalytic activity; thermal annealing; modified TiO₂; band gap
• Online Access: https://www.mdpi.com/2072-666X/9/12/618
• ISSN: 2072-666X

Nitrogen-doped TiO₂ nanotube arrays (N-TNAs) were successfully fabricated by a simple thermal annealing process in ambient N₂ gas at 450 °C for 3 h. TNAs with modified morphologies were prepared by a two-step anodization using an aqueous NH₄F/ethylene glycol solution. The N-doping concentration (0–9.47 at %) can be varied by controlling N₂ gas flow rates between 0 and 500 cc/min during the annealing process. Photocatalytic performance of as-prepared TNAs and N-TNAs was studied by monitoring the methylene blue degradation under visible light (λ ≥ 400 nm) illumination at 120 mW·cm⁻². N-TNAs exhibited appreciably enhanced photocatalytic activity as compared to TNAs. The reaction rate constant for N-TNAs (9.47 at % N) reached 0.26 h⁻¹, which was a 125% improvement over that of TNAs (0.115 h⁻¹). The significant enhanced photocatalytic activity of N-TNAs over TNAs is attributed to the synergistic effects of (1) a reduced band gap associated with the introduction of N-doping states to serve as carrier reservoir, and (2) a reduced electron‒hole recombination rate.