(a)
(b)
Figure 3: XRD patterns of the pure TiO2 nanotube arrays and N-doped TiO2 nanotube arrays annealed at temperatures ranging from 300 to 600°C. Pure TiO2 nanotube array films: (a) before annealing, (b) at 300°C. N-doped TiO2 nanotube array films: (c) 300°C, (d) 400°C, (e) 500°C, and (f) 600°C.