Abstract

The mechanism of CO/N₂ sensitivity of undoped and Pd-doped nanocrystalline tin dioxide was studied in situ by coupled electrical measurements and ¹¹⁹Sn Mössbauer spectroscopy. Nanocrystalline SnO₂ with a grain size of about 6–8 nm was synthesized by the sol–gel method. SnO₂(Pd) ([Pd] = 1 at%) was prepared from SnO₂ by the impregnation method. Electrical properties of SnO₂ and SnO₂(Pd) in the presence of CO/N₂ were studied at a fixed temperature in the range 50 ≤ T ≤ 380 °C. Addition of Pd strongly increases the sensor response to CO over the whole temperature range. It was shown by Mössbauer spectroscopy that two different mechanisms of gas sensitivity can exist in the presence of CO/N₂. The first “low temperature” mechanism (T < 100 °C) may be attributed to redox reactions between surface oxygen and CO catalyzed by PdO_x clusters. The second “high temperature” one (125–380 °C) involves oxygen vacancies diffusing into the bulk of the tin dioxide grains which leads to reversible reduction of Sn(IV) to Sn(II).