Electron spin resonance from annealed titania nanotubes

Titania nanotubes were prepared using a hydrothermal method. Hydrogen titanate nanotubes (H-TNTs) with an anatase phase changed to anatase nanocrystals at about 500 °C, and then a rutile structure at ∼800 °C. A sharp and symmetrical electron spin resonance (ESR) signal (g=2.003), attributed to a single-electron-trapped oxygen-vacancy (SETOV), was obtained at the annealed H-TNTs (T<500 °C). The SETOV signal increased and maximized remarkably at about 400–500 °C. Then, the nanotube structure appeared to be demolished. Yet, when the vacuum-heated H-TNTs were sealed in N2 or Ar ambient, some additional ESR signals appeared besides the SETOV signal. The broad asymmetric ESR signal (g=1.98) was attributed to a surface oxygen vacancy related to the Ti3+ sites in a reduced TiO2 matrix. The vacuum-heated sodium titanate nanotubes (Na-TNTs) showed only the SETOV signal (T<500 °C).