V And N Codoped TiO₂ Nanotube Arrays For Improved Photoelectrochemical And Photocatalytic Properties

Vanadium and nitrogen co-doped TiO2 nanotube array films were fabricated by electrochemical anodization in association with hydrothermal treatment or impregnation process. After V and N co-doping, TiO2 nanotube array films present a higher visible light absorbance. At the same time,codoped TiO2 photocatalysts show remarkably enhanced photocatalytic activity for dye degradation under visible light irradiation and CO2 photoreduction to methane under ultraviolet illumination,respectively. The main work can be divided into the following three parts:(1) Highly ordered TiO2 nanotube arrays(TNTAs) codoped with V and N were synthesized by electrochemical anodization in association with hydrothermal treatment. Scanning electron microscopy(SEM) images indicate that TiO2 nanotubes were hexagonally close packed and highly ordered vertical orientation with an average outer pore diameter of ca. 110 nm, wall thickness of ca. 22 nm and tube length of ca. 11 μm. The appropriate doping amount of V and N does not change the morphology of nanotubes. The peak(101) position in X-ray diffraction(XRD) pattern of the V and N co-doped TNTAs gradually shifted toward a higher diffraction angle. It suggested that V and N ions might be successfully incorporated into the crystal lattice of anatase TiO2. X-ray photoelectron spectroscope(XPS) revealed the existence of interstitial N and V(V4+ and V5+) in V, N codoped samples. Compared with pure TiO2 nanotube arrays, obvious absorption edge red-shifts were observed for V and N codoped samples, which showed stronger photocurrent response and much higher photocatalytic activity for methylene blue(MB)photodegradation under visible light irradiation. Besides, V and N codoped samples would significantly enhance the photocatalytic activity of CO2 reduction.(2) TiO2 nanotube arrays co-doped with vanadium and nitrogen were synthesized by impregnation process. XRD spectra indicate V and N ions were effectively incorporated into the TiO2 lattice. SEM images show V and N codoped TNTAs still possess excellent regular and orderly nanotube array structure. XPS spectroscopy revealed the coexistence of V4+ and V5+ in V and N codoped TiO2 catalytsts,which is beneficial to seperating of charges and improving photocatalytic activity of methyl orange(MO)degradation under visible light irradiation. Added H2O2 can further improve the activity of MO photodegradation because H2O2 can serve as an electron scavenger and a precursor for ?OH radicals.Codoped TiO2 catalysts prepared by impregnation and hydrothermal method exhibit equally excellent photocatalytic activity for dye degradation and CO2 reduction. Besides, the films of codoped catalyst prepared by impregnation method are more stable than those prepared from hydrothermal method.(3) Pt was deposited on the surface of V and N codoped TiO2 nanotube arrays by photoreduction method. SEM and transmission electron microscopy(TEM) images indicate that Pt nanoparticles with size of ca.1-3 nm are uniformly distributed on the nanoporous top surface and underneath nanotube wall.XPS analysis indicated that Pt0 nanoparticles were deposited on the surface of catalysts by photo-reduction process. The uniform distribution of Pt nanoparticles is conducive to the separation of photo-generated electrons and holes. Moreover, the deposition of Pt nanoparticles can not change the nanotube structure and the valence state of doped V and N elements. The photoelectrochemical and photocatalytic experiments proved that the synergistic effect between codoping and Pt modification can promote the generation and transfer of photoelectrons, which is conducive to CO2 methanation.