A combined EXAFS and diffraction study of pure and doped nanocrystalline tin oxide

Nanocrystals of pure, Cu2+ and Fe3+ doped SnO2 have been prepared by a sol-gel route and were investigated by a combination of EXAFS and XRPD measurements, Surface doped samples were also studied, and these were prepared by immersing pure nanocrystals in aqueous solutions of the dopant cations. The XRPD results showed the average size of the crystallites when freshly prepared was 2-3 nm and was not affected by the dopant. The Sn K-edge EXAFS of the pure SnO2 nanocrystals was consistent with their size and suggested the level of disorder in the crystallites was comparable to that in bulk SnO2. The K-edge EXAFS of the dopants showed that up to at least 10 mol % nominal doping of both Cu2+ and Fe3+ ions in sol-gel prepared samples were situated on Sn4+ sites in the SnO2 cassiterite lattice. In the case of the surface doped samples the EXCAFS showed no penetration of the dopant into the SnO2 lattice. The growth of the crystallites on heating was monitored by XRPD and was found to become clearly evident at about 400 degrees C, The effect of the dopants was to lower the growth rate of the nanocrystals. Heating the doped sol-gel prepared samples yielded EXAFS spectra that indicated the dopants gradually moved to surface regions of the crystallites, In the case of the nominally 10 mol % Fe3+ doped sample, heating for 1 h at 900 degrees C caused the precipitation of iron oxide. Heating the surface doped samples up to 900 degrees C did not cause a penetration of the dopant into the SnO2 lattice.