(Keynote) Progress in the Development of Metal Oxide Gas Sensors to Reduce Carbon Footprint

A chemical gas sensor is a device that transfers chemical information (receptor) of a specific sample component or total composition into an analytically useful signal (transducer). Based upon the transducer operating principle, the signals could be the results of optical, electrochemical, mass, calorimetric and/or magnetic properties. The challenge to fix the current issues on cost, sensitivity, selectivity and reliability of a gas sensor, metal oxide based electrochemical gas sensors are much emphasized for the development of suitable sensor materials. As such, electrochemical gas sensors are mainly categorized into potentiometric, amperometric and resistive-type depending upon the analytical electrical signals; V, I and R, respectively.

The real-time detection of gas composition is essential for improving efficiency in industrial process and to lower the greenhouse gas (GHG) emissions. CO₂, one of the major components of GHGs, encounters a challenge on being monitored at real-time under harsh environmental conditions. Spectroscopic and optical techniques based on infrared radiation are commonly used to detect CO₂ at room temperature, however, the in situ monitoring of the gas species by placing such sensors directly at high temperature and aggressive environment is practically incompatible due to their size and stability issues. Na⁺ and Li⁺ ion conducting electrolytes (e.g., Na-β-alumina, NASICON-type), and Na₂CO₃ and BaCO₃ electrodes have been widely employed to fabricate all-solid-state EMF CO₂ sensor.  On the other hand, SnO₂, TiO₂ and WO₃ as semiconductor-based sensors were also used as resistive-type CO₂ sensors. In both cases, the stability and cross sensitivities, however, have remained as major obstacles. Our approach was, therefore, to use perovskite oxides (POs) in semiconductor-based gas sensors, for their high thermal and chemical stability, which in turn improve sensor's reliability and long-term performance. In addition, the doping flexibility in POs allowed us to prepare transition metal-doped perovskites. The critical role of dopant on CO₂ sensing properties is discussed. The future prospect of using nanotech with its ability to precisely control the structure of these materials may guide CO₂ sensors into a whole new level.