Response of SnO2-based sensors to CO under extreme conditions was investigated. It was found that Pd and V2O5loaded SnO2 device (thick film), after a reduction treatment in CO flow, showed reverse response to dilute CO (increase in resistance) under particular operating conditions, i.e., lower temperature below 325 °C, low humidity less than 100 Pa water vapor pressure. The reverse response tended to be more conspicuous as the CO concentration decreased and the coexistent O2 partial pressure increased. Similar reduction-induced reverse response to CO was also observed with Pd-loaded SnO2 device. XPS analysis suggested that the reverse response should be related with the oxidation state of SnO 2.
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