N-Doped TiO₂ Prepared from Microwave-Assisted Titanate Nanotubes (Na_<i>x</i>H_2−<i>x</i>Ti₃O₇): The Effect of Microwave Irradiation during TNT Synthesis on the Visible Light Photoactivity of N-Doped TiO₂

Abstract

This study aimed to characterize N-doped TiO2 prepared by thermally treating microwave-assisted titanate nanotubes (TNTs, NaxH2−xTi3O7) in an Ar/NH3 atmosphere. The effect of intercalated Na(I) within TNTs on the visible light photoactivity and the N-doping mode was investigated as well. By evaluating the performance of photocatalytic oxidation of phenol under the visible region, the photoactivity of N-doped TiO2 prepared from TNTs is 3 times higher than that of N-doped TiO2 prepared from P25 TiO2. Characterizations, including HR-TEM, XRD, XPS, NH3-TPD, UV−vis DRS, and SBET, indicate that the substitutional N-doping mode, the O−Ti−N linkage, is mainly responsible for narrowing the band gap and eventually enhancing the visible light photoactivity. Furthermore, the doping mechanism is significantly dependent on the presence of intercalated Na(I) within TNTs. The O−Ti−N linkage, owing to the substitutional doping, is apparent for TNTs with a low content of intercalated Na(I), whereas the presence of the higher amount of intercalated Na(I) leads to the formation of the Ti−N−O linkage that is considered as an interstitial doping mode. Also, the presence of intercalated Na(I) during the doping process results in the formation of Na2Ti6O13 instead of an inert TiN crystallinity, which is advantageous to enhancing the photoactivity of N-doped TiO2 due to the effect of interphase electron transfer between Na2Ti6O13 and TiO2.