Surface and bulk characterisation of titanium–oxo clusters and nanosized titania particles through 17O solid state NMR

Abstract

For the first time, titanium–oxo based nano-objects have been probed using ¹⁷ O MAS and 3Q-MAS NMR. Three titanium oxo–organo clusters {[Ti ₁₂ O ₁₆ (OPr ⁱ ) ₁₆ ], [Ti ₁₆ O ₁₆ (OEt) ₃₂ ] and [Ti ₁₈ O ₂₂ (OBu ⁿ ) ₂₆ (acac) ₂ ]}, and monodisperse nanoparticles of titania anatase having 20 and 30 Å oxide core diameters, have been characterised by ¹⁷ O NMR. This study shows that in titanium oxo-based compounds, the µ ₂ -O sites are dominated by a significant chemical shift anisotropy, an interaction that is much weaker for the other µ _n -O sites (n = 3, 4, 5). For all µ _n -O sites (n = 2, 3, 4 or 5), the ¹⁷ O NMR linewidths are dominated by chemical shift distribution with a minor contribution from second order quadrupolar broadening. However, depending on the degree of distortion from tetrahedral geometry, the µ ₃ -O sites can also be sensitive to second order quadrupolar effects. Bulk µ ₃ -O and surface oxo species (µ ₃ -O, µ ₂ -O and acac–Ti) present in titania anatase nanoparticles are identified and clearly assigned. The ratio between bulk and surface species decreases as the particle size is increased. The surface reconstruction of the nanoparticles in the presence of ¹⁷ OH ₂ enriched moist air, at room temperature, is demonstrated by ¹⁷ O MAS NMR experiments. From these experiments the µ ₃ -O, µ ₂ -O, Ti–OH, acac–Ti and H ₂ O → Ti surface species were identified.