Chemistry in a confined space: characterization of nitrogen-doped titanium oxide nanotubes produced by calcining ammonium trititanate nanotubes

Ammonium trititanate nanotubes ((NH4)2Ti3O7, abbreviated as NH4TNT) were produced from sodium trititanate nanotubes (Na2Ti3O7, abbreviated as NaTNT) by ion exchange using 1.0 M NH4NO3. Substituting NH4+ for Na+ reduced the band gap energy (Eg) of the trititanate nanotubes. Calcining NH4TNT at 473 K reduced the inter-layer spacing in the nanotube wall, and further reduced the value of Eg, yielding NH4TNT that responded to visible light. As NH4+ cations were intercalated in the small inter-layer space of NH4TNT, calcination at 573 K decomposed NH4+ (NH4TNT → NH3 + HTNT) and produced inside the nanotube wall NH3 gas at a high pressure, which fractured and thereby shortened the nanotubes. Calcination at 573 K also caused a phase transformation from hydrogen trititanate to TiO2. Calcination at 673 K induced the dehydrogenation of NH3 molecules that were confined to the nanotube wall, producing interstitial NH2 species. Calcinations at between 573 and 673 K resulted in the formation of N-TiO2 (B) nanotubes and N-anatase nanotubes, which have a narrow band gap (2.96 ∼ 2.76 eV) and respond to visible light. Further calcination at ≥ 773 K caused the loss of N species and the disappearance of the tubular pore of the nanotubes. The activities of N-doped TiO2 nanomaterials that were calcined at various temperatures in degrading methylene blue followed the order: 673 > 573 > 473 >773 > 873 K. The active N species in these N-doped TiO2 are molecular nitrogen species, including NH4+ and NH3 at high concentrations (3.8 ∼ 1.2 atomic %) at 473 and 573 K, and NH2 at a low concentration (0.4 ∼ 0.2 atomic %) at 673 and 773 K. The nature and concentration of the N species, surface area, the crystallinity and the crystalline composition of the material govern the photocatalytic activity of N-doped TiO2 that is prepared by calcining the NH4TNT.