Atypical humidity-enhanced NO₂ sensing in WO₃ via Zn doping and visible-light activation: Mechanistic elucidation and application to breath analysis

Humidity remains a persistent obstacle for metal oxide semiconductor (MOS) gas sensors, often slowing surface reactions and reducing sensitivity, thereby limiting their use in breath analysis. Here, we demonstrate a new paradigm in which humidity is transformed from a limitation into a co-activating factor for highly sensitive, room-temperature NO₂ sensing, achieved by combining Zn doping with visible-light activation in WO₃ nanoparticles. Systematic variation of doping levels, illumination wavelengths, and relative humidity (RH) conditions reveals an atypical regime in which high RH markedly amplifies NO₂ responses and enhances recovery under specific visible-light activation. The optimized material delivers an ultralow detection limit (∼1 ppb) along with outstanding selectivity, stability, and reproducibility, enabling reliable ppb-level detection in real breath samples and highlighting its potential for asthma diagnosis. Mechanistic studies link band-gap narrowing, oxygen vacancy enrichment, prolonged carrier lifetimes, and hydroxyl radical formation induced by Zn doping to the observed humidity-enhanced performance under visible-light illumination. This impurity-doping-assisted, humidity-enhanced photoactivation strategy offers a practical and generalizable route to high-performance MOS gas sensing in moisture-rich environments, including portable breath diagnostics and environmental monitoring.