Relationship between N-doping induced point defects by annealing in ammonia and enhanced thermal stability for anodized titania nanotube arrays

摘要

Annealing in ammonia slows down the anatase-rutile phase transformation (ARPT) of TiO_2 nanotube arrays (NTs) by anodization as compared with that of NTs annealed in oxygen. This improves the thermal stability of NTs indicated by SEM, XRD, Raman spectroscopy. The surface states of NTs annealed in ammonia identified by XPS as N 1s, 0 1s and Ti 2p indicate that the retarded ARPT and enhanced thermal stability are closely related to nitrogen interstitials. At temperature below 550 °C, the chemisorbed NH_3 for γ-N on the surface of NTs is predominant. In contrast, at annealing temperature above 550 "C, the oxygen vacancies compensating the charge balance of substitutional nitrogen result in the acceleration of ARPT and subsequently the collapse of nanotubular structure. Combining XPS with UV-vis diffuse reflection spectra, it is found that substitutional N impurities significantly enhance the visible light absorption of NTs.