Enhanced photocatalytic performance of nitrogen-doped TiO₂ Nanotube arrays using a simple annealing process

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.