Structural Characterization of Nanocrystalline Sb-Doped SnO2 Xerogels by Multiedge X-ray Absorption Spectroscopy

Multiedge XAS data are presented for Sb-doped SnO2 xerogels dried at 200 °C and fired at 550 °C, aiming to determine the location of antimony doping in the host matrix and then to understand the role that the intentional impurity plays in the structure and properties of the tin oxide based materials. Xerogel processing at 200 °C leads to the original trivalent antimony, used for the xerogel preparation to the SbV oxidation state, mainly for low doping levels ([Sb] ≤ 4 atom %). As the doping level increases, a significant amount of antimony remains in the trivalent oxidation state. Upon firing at 550 °C, the antimony is present mainly as SbV, independent of the doping level. The analysis of EXAFS data recorded at the Sn and Sb K edges leads to the conclusion that doping with Sb for a level of less than 4 atom % favors crystallite growth, concomitant with a strong dominance of the SbV oxidation state. Besides, the pentavalent antimony is located at internal sites of the SnO2 nanocrystallites. The EXAFS spectra of the higher ([Sb] > 4 atom %) doped samples can always be fitted by a linear combination of the spectra corresponding to the SbV site in solid solution and the spectra corresponding to the sample where SbIII is chemically grafted at the surface of the SnO2 crystallite. This SbIII segregation also agrees with the low electrical conductivity reported for sol−gel deposited films, because it generates a barrier at the grain boundary, inducing a very high electron scattering and, thus, a low electron mobility.