Novel Operation Strategy to Obtain a Fast Gas Sensor for Continuous ppb-Level NO₂ Detection at Room Temperature Using ZnO—A Concept Study with Experimental Proof

Abstract

A novel sensor operation concept for detecting ppb-level NO2 concentrations at room temperature is introduced. Today’s research efforts are directed to make the sensors as fast as possible (low response and recovery times). Nevertheless, hourly mean values can hardly be precisely calculated, as the sensors are still too slow and show baseline drifts. Therefore, the integration error becomes too large. The suggested concept follows exactly the opposite path. The sensors should be made as slow as possible and operated as resistive gas dosimeters. The adsorption/desorption equilibrium should be completely shifted to the adsorption side during a sorption phase. The gas-sensitive material adsorbs each NO2 molecule (dose) impinging and the sensor signal increases linearly with the NO2 dose. The actual concentration value results from the time derivative, which makes the response very fast. When the NO2 adsorption capacity of the sensor material is exhausted, it is regenerated with ultraviolet (UV) light and the baseline is reached again. Since the baseline is newly redefined after each regeneration step, no baseline drift occurs. Because each NO2 molecule that reaches the sensor material contributes to the sensor signal, a high sensitivity results. The sensor behavior of ZnO known so far indicates that ZnO may be suitable to be applied as a room-temperature chemiresistive NO2 dosimeter. Because UV enhances desorption of sorbed gas species from the ZnO surface, regeneration by UV light should be feasible. An experimental proof demonstrating that the sensor concept works at room temperature for ppb-level NO2 concentrations and low doses is given.

Keywords

• resistive gas dosimeter
• room-temperature gas sensing
• ZnO
• UV-supported NO₂ sensing
• air quality monitoring