Some new insights into the sensing mechanism of palladium promoted tin (IV) oxide sensor

The combination of techniques including switching experiments, temperature programed reduction and in situ neutron scattering-conductivity are used to investigate the sensing mechanism of 1% Pd/SnO2 toward hydrogen-containing gas mixtures. In particular, the use of the in situ neutron scattering-con ductivity for the first time allows the simultaneous monitoring of electric al conductivity and inelastic neutron scattering spectra of the sensor mate rial. Direct evidence is obtained on a reversible migration of hydrogenic s pecies from and to the metal and the underlying tin oxide surface, i.e., re versible hydrogen spillover. As a result of this spillover, a dramatic chan ge in electrical conductivity of the Pd doped tin oxide material is observe d. We confirm, in accordance with the known mechanism, that the change of c onductivity is based upon the creation or destruction of negatively charged adsorbed oxygen species on the sensor surface. In addition, we report a ne w but important sensing mechanism, the spillover hydrogen species behaving like a shallow donor to the semiconductor oxide as the direct source of con ductivity occurs concurrently with the known mechanism.