Engineered Porosity ZnO Sensor Enriched with Oxygen Vacancies Enabled Extraordinary Sub-ppm Sensing of Hydrogen Sulfide and Nitrogen Dioxide Air Pollution Gases at Low Temperature in Air

We report the results of a zinc oxide (ZnO) low-power micro sensor for sub-ppm detection of NO2 and H2S in air at 200°C. NO2 emission is predominantly produced by combustion processes of fossil fuels while coal-fired power plants are the main emitter of H2S. Fossil fuels (oil, natural gas, and coal) combined contained 74% of USA energy production in 2023. It is foreseeable that the energy industry will utilize fossil-based fuels more in the ensuing decades despite the severe climate crises. Precise NO2 and H2S sensors will contribute to reduce the detrimental effect of the hazardous emission gases in addition to the optimization of the combustion processes for higher output. Fossil fuel industry and the Solid-oxide fuel cells (SOFCs) are exceptional examples of energy conversion-production technologies that will profit from advances in H2S and NO2 sensors. Porosity and surface activity of metal oxide semiconductors (MOS) based sensors are both vital for sensing at low temperatures. Oxygen vacancies (V_O^(••)) act as surface active sites for target gases, while porosity enables target gases to come in contact with a larger MOS area for sensing. We were able to create an open porosity network throughout the ZnO microstructure and simultaneously achieve an abundance of oxygen vacancies by using a heat treatment procedure. Surface chemistry and oxygen vacancy content in ZnO were examined using XPS and AES. SEM was used to understand the morphology of the unique characteristics of distinctive grain growth during heat treatment. Electrical resistivity measurements were completed. Valance band was examined by UPS. Engineered Porosity approach allowed the entire ZnO act as an open surface together with creation of abundant oxygen vacancies (V_O^(••)). NO2 detection is challenging since both oxygen (O2) and NO2 are oxidizing gases and they coexist in combustion environments. Engineered porosity ZnO micro sensor detected sub-ppm NO2 under O2 interference affect mimicking realistic sensor operation conditions. Engineered Porosity ZnO performed better than previous literature findings for H2S and NO2 detection. The exceptionally high in sensor response attributed to the high number of oxygen vacancies (V_O^(••)) and porosity extending through the thickness of the ZnO with high degree of tortuosity. These features enhance gas adsorption and diffusion via porosity leading to high sensor response.