Enhanced Photocatalytic Performance of Nitrogen-Doped TiO(2) Nanotube Arrays Using a Simple Annealing Process

Nitrogen-doped TiO(2) nanotube arrays (N-TNAs) were successfully fabricated by a simple thermal annealing process in ambient N(2) gas at 450 °C for 3 h. TNAs with modified morphologies were prepared by a two-step anodization using an aqueous NH(4)F/ethylene glycol solution. The N-doping concentration (0–9.47 at %) can be varied by controlling N(2) 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.