Detecting atmospheric CO2 with a sensitive sensor

Take a deep breath. Chances are that air filling your lungs is impure and polluted with deadly gases like carbon dioxide. According to the World Health Organisation, every year around two million people die prematurely due to air pollution.

The air we breathe consists mostly of nitrogen and oxygen and minute quantities of carbon dioxide (CO2) and argon. Outdoor ambient air normally has about 250-350 parts per million (ppm) of CO2. Breathing in air containing more than 1000 ppm of CO2 can cause headache, drowsiness and nausea and anything greater than 10000 ppm of CO2 can cause permanent brain damage, coma and even death.

Conventional metal oxide semiconductor gas sensors can detect CO2 in the higher range. However, for detecting atmospheric CO2 concentration, which is in the range of 400 ppm, a highly sensitive detector is required. While optical sensors based on absorption spectroscopy can be very accurate, they are highly expensive and bulky, precluding large scale deployment.

Enhanced performance
Now, Professor Navakanta Bhat and his team at Indian Institute of Science, Bengaluru, have devised one such sensor that is highly accurate and sensitive and capable of detecting CO2 in volumes as low as 400 ppm. This novel sensor is made by placing a thin layer of metal oxides (oxides of barium, titanium and copper) which possess properties similar to semiconductors and hence called ‘semiconductor metal oxide’ sensor.

These gas sensors detect gases by a chemical reaction that takes place when the gas comes in direct contact with the sensor. Because of the chemical reaction, the electrical properties also change, thus enabling the detection of the gas by measuring the resistance values of the metal oxide sensor. The novel sensor forms a distributed ‘heterojunction’ diode, which is responsible for improved sensitivity for small volumes of CO2. A heterojunction is the interface between any two solid-state materials, in this case the surface of barium-titanium oxide and copper oxide. To further enhance the detection performance, an extensive study was conducted and it was found that addition of Silver on metal oxide catalyses the detection process. The optimum operating temperature for the sensor with silver doping was found to be at 250°C, for maximum sensor response of 19% for 350 ppm and 80% for 1000 ppm, which is the best obtained so far among the semiconductor metal oxide gas sensors.

Various experiments have also been performed to confirm that the new sensor can uniquely sense CO2 among the mixture of other gasses like CO, NO2, SO2, proving the robustness of the sensor. To accurately detect the CO2, the sensor has to sample the air for about 1.5 minutes and wait for 2 minutes until the sensor is ready for analysis of the new specimen.

Currently available CO2 sensors are expensive and bulky. Moreover, many gas sensors can detect more than 1000 ppm of CO2, but not lower volumes. This sensor is a much needed innovation in monitoring atmospheric CO2 concentration, one of the most critical air pollutants. So, a sensor of this kind perfectly qualifies for pollution monitoring applications.

“We are hoping to deploy such sensors on every traffic intersection of a city like Bengaluru. This will enable the citizens to know about the quality of the air that they breathe, hopefully by syncing their cell phones to the sensor. This awareness could in turn lead to appropriate interventions to reduce pollution,” says Navakanta. The findings have been published in a research paper, ‘Optimisation of RF Sputtered Ag-Doped BaTiO3-CuO Mixed Thin Film as Carbon Dioxide Sensor for Environmental Pollution Monitoring Application’ in IEEE Sensors Journal.