Optimizing Ozone Sensing Performance of Co-Doped ZnO Thin Films via Defect Engineering

Semiconducting metal oxides (SMOs) have been extensively employed as sensing materials for the detection of various toxic gases. Among them, zinc oxide (ZnO) is a multifunctional n-type semiconductor that exhibits potential properties for gas-sensing, particularly toward oxidizing gases. In this study, cobalt-doped ZnO thin films were synthesized via spray pyrolysis and investigated for their application as resistive ozone sensors. All thin films exhibited a hexagonal wurtzite structure with no detectable secondary phases. Furthermore, incorporation of Co 2 + ions into the ZnO lattice was confirmed. Photoluminescence measurements revealed a broad blue emission band associated with shallow-donor defects, which was found to correlate directly with an increased density of active sites on the film surface. Electrical measurements showed that the thin film containing 2.5 mol% Co exhibited a 37-fold enhancement in O₃ sensor response at ppb levels compared to pristine ZnO, attributed to shallow-donor -defect-mediated adsorption mechanisms. Furthermore, thermal annealing under reducing and oxidizing atmospheres was deliberately used as an independent defect-modulation strategy, confirming that the enhancement in O₃ sensor response is governed by blue-luminescence-related defects. These findings suggest blue luminescence as a potential optical parameter for optimizing ZnO-based ozone sensors, offering a rapid and non-destructive approach for sensor design. • Co-doped ZnO thin films exhibit a nanograined surface morphology. • An optimal Co concentration (2.5 mol%) enhances ozone sensor response at ppb levels. • The blue luminescence band is associated with an increased concentration of active sites promoting O₃ chemisorption. • Blue luminescence intensity is proposed as a practical optical descriptor correlated with O₃ sensing performance.