Correlation between XPS, Raman and TEM measurements and the gas sensitivity of Pt and Pd doped SnO2 based gas sensors

Nanocrystalline thick-film SnO2 sensors with different dopants were fabricated by an optimized screen printing process and subsequent annealing. Powders were used as starting materials which were prepared by a wet chemical process from SnCl4. Microanalysis was performed of both, the precursors and the final sensor materials with their different annealing conditions. Gas sensing tests with CO, CH4 and NO2 in air with controlled humidity were correlated with results from X-ray photoemission spectroscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). As an interesting result, the distribution of the transition metal dopants Pd and Pt (as deduced from TEM and XPS data) rules out the existence of metallic clusters or even atoms in the metallic state at the surface. This finding does not allow to explain the sensor effects on SnO2 based materials as usually done by means of spill-over effects or Fermi energy control.