Sub-ppb Sensing Level Hydrogen Sulphide at Room Temperature Using Doped Indium Oxide Gas Sensors

Abstract

Hydrogen sulfide (H2S) is a highly corrosive, harmful, and toxic gas. Whereas 130 ppb is the threshold for maximum safety exposure level. Thus, it has become mandatory to develop highly sensitive sensors operating at room temperature to detect low levels of H2S. Sensors based on electrochemical measurement principles, semiconducting metal oxides or p–n heterojunctions have showed promising results to detect H2S gas. To date, drawbacks have boon reported due to their extended response times, high operating temperatures, limited selectivity to the gas and/or sensitivity to humidity changes. Efforts have been made to develop sensors based on metal oxides operable at room-temperature for H2S detection.Here, we describe novel flexible H2S gas sensors based on optimized composite mixture of indium oxide (In2O3), copper acetate (CuAc), graphite (Gt) and polystyrene (PS). While we employed Gt in sensor’s structure to reduce sensor’s resistance to ~ 0.5 MΩ, we used PS and enhance sensor’s integrity as well as the adhesiveness of the sensing layer on the top of carbon electrodes. Whereas we applied In2O3 as the active material in sensors due to its promising performance to detect ~ 100 ppb gas concentration. We used CuAc as activator when it converts to the form of copper sulfide (CuS) due to its high conductivity (10 S/cm). Hypothetically, upon exposing sensors to the H2S gas, In2O3 converts spontaneously to the metallic form In2S3 (eq.1). Alternately, CuAc plays two critical roles in the sensor composition. First, CuS formation drops sensor’s resistance significantly due its high conductivity (eq.2). At this point, In2S3 and CuS can form a continues metallic layer, thus it increases sensor’s conductivity. Second, CuS may play a similar role as same as noble metals, which can cause the formation the structure of noble-doped metal oxide and ease oxygen molecules adsorption/ionization on sensor’s surface (eq.3,4). This may facilitate oxidizing and/or adsorption H2S on the sensor’s surface (eq.5,6), which consists of releasing electrons to sensors and reduce sensors resistance dramatically. The developed sensors have showed high selectivity toward H2S gas, high sensitivity to H2S concentration of less than 100 ppb, fast detection time less than 60 seconds, and high resistivity to humidity changes. Figure 1