Abstract
An FET-type CO 2 sensor operative at room temperature was developed by stacking a powder mixture of metal carbonate (auxiliary phase)—indium tin oxide (ITO, sensing electrode) as well as Na +-exchanged cation exchange membrane (ionic conductor gate) on an FET chip. Both ITO and the cation exchange membrane were indispensable to obtain stable response to CO 2 at room temperature, while the auxiliary phase was optimized to be Li 2CO 3–BaCO 3 (1:3 in molar ratio) from the stability of response against the disturbance by humidity. The device, Li 2CO 3–BaCO 3–ITO (1:3:1)/Na +-exchanged cation exchange membrane/FET, exhibited excellent CO 2-sensing characteristics at 30 °C. The response in gate-source voltage ( V GS) was linear to the logarithm of CO 2 concentration, with its slopes indicating n=2.0 where n is the number of reaction electrons involved in the electrochemical reduction per CO 2. The Nernst’s correlation was fairly stable against a change in RH between 30 and 70%. The times for 90% response and 90% recovery to switching-on and -off 3000 ppm CO 2 were 1 and 2 min, respectively.
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