Chemistry in a confined space: characterization of nitrogen-doped titanium oxidenanotubes produced by calciningammonium trititanate nanotubes

Abstract

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 (E_g) of the trititanate nanotubes. Calcining NH₄TNT at 473 K reduced the inter-layer spacing in the nanotube wall, and further reduced the value of E_g, 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.