Ammonium trititanate nanotubes ((NH4)2Ti 3O7, 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-TiO 2 (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 NH 4+ 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.
All Science Journal Classification (ASJC) codes
- Materials Chemistry