Molecular nitrogen in N-doped TiO2 nanoribbons

Abstract

The nitrogen doping of TiO₂ nanoribbons during the thermal transformation of hydrogen titanate nanoribbons (HTiNRs) between 400 and 650 °C in a dynamic ammonia atmosphere was investigated using X-ray photoelectron spectroscopy (XPS), transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (NEXAFS-TXM), X-ray diffraction (XRD) and electron paramagnetic resonance measurements (EPR). Comprehensive structural characterizations have revealed that for a calcination temperature of 400 °C, the HTiNRs transform into pure monoclinic TiO₂ β-phase (TiO₂-B) whereas at higher calcination temperatures (580 and 650 °C) a mixture of TiO₂-B and anatase is obtained. XPS and EPR results clearly reveal the nitrogen doping of TiO₂ nanoribbons and that, depending on the calcination temperature, nitrogen atoms occupy interstitial and substitutional sites. Moreover, in samples calcined at 580 and 650 °C the presence of N₂-like species in the HTiNRs was detected by NEXAFS-TXM. These species are trapped in the HTiNRs structure. EPR measurements upon light illumination have disclosed the generation of photoexcited states which implies that nitrogen has an important effect on the electronic structure of N-doped TiO₂.