  Molybdenum and nitrogen co-doped $TiO_2$ nanotube arrays (TNAs) were prepared
by anodizing in association with hydrothermal treatment. As-prepared Mo and N
co-doped TNAs were characterized by field emission scanning electron
microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and
ultraviolet-visible light diffuse reflection spectroscopy. Besides, the
photocatalytic activity of Mo and N co-doped TNAs for the degradation of
methylene blue (denoted as MB) under visible light irradiation was evaluated.
It was found that N in co-doped TNAs coexists in the forms of N-Ti-O and
N-O-Ti, while Mo exists as $Mo^{6+}$ by substituting Ti in the lattice of
$TiO_2$. In the meantime, Mo and N co-doping extends the absorption of TNAs
into the whole visible light region and results in remarkably enhanced
photocatalytic activity for the degradation of MB under visible light
irradiation. This could be attributed to the synergetic effect between
Mo-doping and N-doping. Namely, Mo and N passivated co-doping produces new
states in TNAs, thereby narrowing the bandgap and decreasing the recombination
rate of electrons and holes. As a result, the visible light absorption and
photocatalytic activity of TNAs is greatly increased. Furthermore, Mo ions with
multiple valences in all co-doped samples could act as trapping sites to
effectively decrease the recombination rate of electrons and holes, which also
contribute to improved photocatalytic activity of Mo and N co-doped TNAs.
Keywords: TiO2 Nanotube Arrays, Mo, N, Co-Doping, Photocatalytic Activity.
