Near edge X-ray absorption fine structure measurements (XANES) and extended x-ray absorption fine structure measurements (EXAFS) of the valence stateand coordination of antimony in doped nanocrystalline SnO2

Colloids of nanocrystalline tin dioxide containing 9.1 at. % and 16.7 at. % antimony have been prepared by the coprecipitation method. High-resolution transmission electron microscopy (TEM) images show crystalline particles i n the 2-6 nm size regime. X-ray powder diffraction patterns of nanocrystall ine powders obtained by drying the colloids and heating to 100 degrees C in dicate the same rutile lattice structure known from bulk SnO2. On heating t o 500 degrees C in air, the nanocrystalline powder shows a slight increase in particle size but especially a change in color from yellowish to bluish which is accompanied by the development of n-type conductivity. The coordin ation of antimony in the SnO2 nanocrystallites has been investigated by ext ended x-ray absorption fine structure measurements (EXAFS) at the Sb K-edge at 5 K while its valence state was determined by near edge x-ray absorptio n fine structure measurements (XANES) at the Sb L-1 edge. The Sb higher nei ghbor shell distances in the doped material differ from the corresponding d istances in Sb2O3 or Sb2O5 but are identical to those in tin dioxide, indic ating that antimony is almost completely incorporated into the tin dioxide lattice despite the high doping level. XANES measurements reveal that a lar ge fraction of Sb-III employed during the synthesis is already oxidized to Sb-V at low temperatures. On the basis of these observations, a two-step mo del for the formation of n-conductive Sb-doped SnO2 nanocrystals is given a nd quantitatively discussed with respect to the data. (C) 2000 American Ins titute of Physics. [S0021-9606(00)70208-X].