Role of indium in enhancing trace-level NO₂ gas-sensing performance of spray-pyrolyzed CuO nanostructures for air-quality monitoring

Chemiresistive gas sensors still suffer from fast response/recovery kinetics at low operating temperatures when detecting trace-level analytes. The work systematically describes the effect of indium incorporation on NO₂ sensing behavior in spray pyrolysis-prepared CuO thin films. The 1% indium-containing film shows a response time of ∼7 s and recovery time of ∼35 s, along with a response of 104% to 4 ppm NO₂ at an operating temperature of 180 °C. The enhanced response kinetics is linked to dopant-induced defect chemistry and oxygen vacancy distribution, which controls the charge transport and gas-surface interactions in the CuO matrix. In contrast, higher indium addition (3% and 5%) induces a transition in the dominant electronic character of the films, leading to the formation of n-p heterojunctions with a consequent change in sensing kinetics upon NO₂ exposure. The results corroborate controlled indium addition to tune the defect states and electronic structure in CuO for obtaining metal oxide gas sensors with pre-defined performance characteristics.