Boosting CO2 sensitivity via multiphase ZnO heterojunctions and surface-engineered silicon pyramids

The development of low-temperature and high-sensitivity CO₂ gas sensors is crucial for applications in environmental monitoring and indoor air quality control. In this study, we report a ZnO-based heterostructure sensor fabricated on silicon pyramid arrays to enhance CO₂ detection performance. Vertically aligned ZnO nanorods were synthesized on etched Si substrates via hydrothermal methods, followed by the formation of ZnSnO₃ and Zn₂SnO₄ heterostructures through corrosion and hydrothermal reactions. XRD and EDS analyses confirmed the formation of composite oxide phases, and energy band diagrams revealed favorable n–n type heterojunctions at the ZTO/ZnO interface. Five types of devices were systematically compared, with the ZnSnO₃-ZnO and Zn₂SnO₄-ZnO sensors demonstrating high sensitivities of 365% and 398%, respectively, toward 2500 ppm CO₂ at an optimal operating temperature of 150 °C. These sensors also exhibited excellent stability over 30 days, minimal cross-sensitivity to common interfering gases (ethanol, acetone, hydrogen), and reduced humidity interference under controlled relative humidity (20–80%). Compared with previously reported metal oxide sensors, the proposed structures offer superior sensitivity, fast response/recovery, and reliable low-temperature operation. This work presents a scalable and material-efficient strategy for the fabrication of selective and stable CO₂ sensors suitable for real-world applications.