SELECTIVITY AND COMPOSITION DEPENDENCE OF RESPONSE OF GAS-SENSITIVE RESISTORS .2. HYDROGEN-SULFIDE RESPONSE OF CR-2-XTIXO3+Y

Titanium-substituted chromium oxides, Cr2-xTixO3+y, form solid solutions in the range 0 less than or equal to x less than or equal to 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.