Development of FET-type CO₂ sensor operative at room temperature

An FET-type CO₂ 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 CO₂ at room temperature, while the auxiliary phase was optimized to be Li₂CO₃-BaCO₃ (1:3 in molar ratio) from the stability of response against the disturbance by humidity. The device, Li ₂CO₃-BaCO₃-ITO (1:3:1)/Na⁺-exchanged cation exchange membrane/FET, exhibited excellent CO₂-sensing characteristics at 30°C. The response in gate-source voltage (V _GS) was linear to the logarithm of CO₂ concentration, with its slopes indicating n = 2.0 where n is the number of reaction electrons involved in the electrochemical reduction per CO₂. 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 CO₂ were 1 and 2 min, respectively.