Structural studies of silica- and alumina-pinned nanocrystalline SnO₂

Sol-gel-prepared nanocrystalline tin (IV) oxide has been characterized by magic angle spinning nuclear magnetic resonance (MAS NMR) in a pure form and also with silica and alumina pinning phases present. The Sn-119 MAS NMR lineshape was observed to broaden as the SnO2 nanocrystal sizes decreased, a reflection of the increasingly important contribution of the more disordered surface region. A second local Sri environment other than the cassiterite phase was observed, probably an orthorhombic phase that has previously been observed in nanoparticles of SnO2. The silica and alumina pinning particles were characterized by Si-29 and Al-27 MAS NMR. The silica pinning particles retained a high degree of disorder over the range of annealing temperatures, whereas alumina crystallized at similar to 1000 degrees C. A possible explanation for this difference is that since the alumina pinning particles are less successful at restricting nanocrystal growth than the silica particles, they may aggregate together and combine, hence they are larger, less constrained particles