Selectivity and composition dependence of response of gas-sensitive resistors. Part 2.—Hydrogen sulfide response of Cr2 –xTixO3 +y

Abstract

Titanium-substituted chromium oxides, Cr_{2 –x}Ti_xO_{3 +y}, form solid solutions in the range 0 ⩽x⩽ 0.4, with surface enrichment of Ti. The electrical behaviour of these compounds was consistent with charge compensation by chromium vacancies. The conductance of porous, small-grain-size specimens could be interpreted as surface trap-limited, with O_ads^– as the surface species. Both molecular and dissociative chemisorption of H₂S was observed. Exposure to H₂S at low concentration in air resulted in the formation of sulfate groups at the surface as well as surface hydroxylation. There was a range of binding energy of the surface sulfate: strongly bound sulfate caused a permanent, temperature-dependent conditioning of the surface characterized by a permanent change in the air baseline resistance after an initial dose of H₂S. The origin of the gas response of the electrical resistance was attributed to the surface reaction of H₂S with O_ads^– to form adsorbed sulfate, which desorbs as sulfur dioxide. The electrical response was repeatable and stable.